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3 Getting to know your Programming
Environment
3.1 Objectives
In this section, we will be discussing on how to write, compile and run Java programs.
There are two ways of doing this, the first one is by using a console and a text editor.
The second one is by using NetBeans which is an Integrated Development
Environment or IDE.
At the end of the lesson, the student should be able to:
• Create a Java program using text editor and console in the Linux (Ubuntu Dapper)
environment
• Differentiate between syntax-errors and runtime errors
• Create a Java program using NetBeans
3.2 Introduction
An IDE is a programming environment integrated into a software application that
provides a GUI builder, a text or code editor, a compiler and/or interpreter and a
debugger.
This tutorial uses Ubuntu Dapper as the operating system. Make sure that before you do
this tutorial, you have installed Java and NetBeans in your system. For instructions on
how to install Java and NetBeans, please refer to Appendix A. For the Windows XP
version of this section, please refer to Appendix B.
Before going into details, let us first take a look at the first Java program you will be
writing.
3.3 My First Java Program
public class Hello
{
/**
* My first java program
*/
public static void main(String[] args) {
//prints the string "Hello world" on screen
System.out.println("Hello world!");
}
}
Before we try to explain what the program means, let's first try to write this program in a
file and try to run it.
Introduction to Programming I 29](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-29-320.jpg)
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Step 5: Compiling your program
Now, the next step is to compile your program. Go to the Terminal window we just
opened a while ago.
Typically, when you open the terminal window, it opens up and takes you directly to
what is called your home folder. To see what is inside that home folder, type ls and
then press ENTER. What you will see is a list of files and folders inside your home folder.
Now, you can see here that there is a folder named "MYJAVAPROGRAMS" which we have
created a while ago, and where we saved our Hello.java program. Now let's go inside
that directory.
To go inside a directory, you type in the command: cd [directory name]. The "cd"
command stands for, change directory. In this case, since the name of our directory is
MYJAVAPROGRAMS, you type in: cd MYJAVAPROGRAMS
Introduction to Programming I 36
Figure 3.6: Changing the Directory](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-36-320.jpg)
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To compile a Java program, we type in the command: javac [filename]. So in this
case, type in: javac Hello.java.
During compilation, javac adds a file to the disk called [filename].class, or in this case,
Hello.class, which is the actual bytecode.
Introduction to Programming I 38
Figure 3.8: Compiling Java File](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-38-320.jpg)
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Step 6: Running the Program
Now, assuming that there are no problems during compilation (we'll explore more of the
problems encountered during compilation in the next section), we are now ready to run
your program.
To run your Java program, type in the command: java [filename without the
extension], so in the case of our example, type in: java Hello
You can see on the screen that you have just run your first Java program that prints the
message, "Hello world!".
Introduction to Programming I 39
Figure 3.9: Running Class File](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-39-320.jpg)
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3.6 Exercises
3.6.1 Hello World!
Using NetBeans, create a class named: [YourName]. The program should output on the
screen:
Welcome to Java Programming [YourName]!!!
3.6.2 The Tree
Using NetBeans, create a class named: TheTree. The program should output the
following lines on the screen:
I think that I shall never see,
a poem as lovely as a tree.
A tree whose hungry mouth is pressed
Against the Earth’s sweet flowing breast.
Introduction to Programming I 55](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-55-320.jpg)
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4 Programming Fundamentals
4.1 Objectives
In this section, we will be discussing the basic parts of a Java program. We will start by
trying to explain the basic parts of the Hello.java program introduced in the previous
section. We will also be discussing some coding guidelines or code conventions along the
way to help in effectively writing readable programs.
At the end of the lesson, the student should be able to:
• Identify the basic parts of a Java program
• Differentiate among Java literals, primitive data types, variable types ,identifiers
and operators
• Develop a simple valid Java program using the concepts learned in this chapter
4.2 Dissecting my first Java program
Now, we'll try to the dissect your first Java program:
public class Hello
{
/**
* My first java program
*/
public static void main(String[] args) {
//prints the string "Hello world" on screen
System.out.println("Hello world!");
}
}
The first line of the code,
public class Hello
indicates the name of the class which is Hello. In Java, all code should be placed inside
a class declaration. We do this by using the class keyword. In addition, the class uses an
access specifier public, which indicates that our class in accessible to other classes from
other packages (packages are a collection of classes). We will be covering packages and
access specifiers later.
The next line which contains a curly brace { indicates the start of a block. In this code,
we placed the curly brace at the next line after the class declaration, however, we can
also place this next to the first line of our code. So, we could actually write our code as:
public class Hello
{
or
public class Hello {
Introduction to Programming I 56](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-56-320.jpg)
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The next three lines indicates a Java comment. A comment is something used to
document a part of a code. It is not part of the program itself, but used for
documentation purposes. It is good programming practice to add comments to your
code.
/**
* My first java program
*/
A comment is indicated by the delimiters “/*” and “*/”. Anything within these delimiters
are ignored by the Java compiler, and are treated as comments.
The next line,
public static void main(String[] args) {
or can also be written as,
public static void main(String[] args)
{
indicates the name of one method in Hello which is the main method. The main method
is the starting point of a Java program. All programs except Applets written in Java start
with the main method. Make sure to follow the exact signature.
The next line is also a Java comment,
//prints the string "Hello world" on screen
Now, we learned two ways of creating comments. The first one is by placing the
comment inside /* and */, and the other one is by writing // at the start of the
comment.
The next line,
System.out.println("Hello world!");
prints the text “Hello World!” on screen. The command System.out.println(), prints the
text enclosed by quotation on the screen.
The last two lines which contains the two curly braces is used to close the main method
and class respectively.
Coding Guidelines:
1. Your Java programs should always end with the .java extension.
2. Filenames should match the name of your public class. So for example, if the name
of your public class is Hello, you should save it in a file called Hello.java.
3. You should write comments in your code explaining what a certain class does, or
what a certain method do.
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4.4 Java Statements and blocks
A statement is one or more lines of code terminated by a semicolon. An example of a
single statement is,
System.out.println(“Hello world”);
A block is one or more statements bounded by an opening and closing curly braces that
groups the statements as one unit. Block statements can be nested indefinitely. Any
amount of white space is allowed. An example of a block is,
public static void main( String[] args ){
System.out.println("Hello");
System.out.println("world");
}
Coding Guidelines:
1. In creating blocks, you can place the opening curly brace in line with the statement,
like for example,
public static void main( String[] args ){
or you can place the curly brace on the next line, like,
public static void main( String[] args )
{
2. You should indent the next statements after the start of a block,for example,
public static void main( String[] args ){
System.out.println("Hello");
System.out.println("world");
}
Introduction to Programming I 59](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-59-320.jpg)
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4.9 Variables
A variable is an item of data used to store state of objects.
A variable has a data type and a name. The data type indicates the type of value that
the variable can hold. The variable name must follow rules for identifiers.
4.9.1 Declaring and Initializing Variables
To declare a variable is as follows,
<data type> <name> [=initial value];
Note: Values enclosed in <> are required values, while those values enclosed in [] are
optional.
Here is a sample program that declares and initializes some variables,
public class VariableSamples
{
public static void main( String[] args ){
//declare a data type with variable name
// result and boolean data type
boolean result;
//declare a data type with variable name
// option and char data type
char option;
option = 'C'; //assign 'C' to option
//declare a data type with variable name
//grade, double data type and initialized
//to 0.0
double grade = 0.0;
}
}
Coding Guidelines:
1. It always good to initialize your variables as you declare them.
2. Use descriptive names for your variables. Like for example, if you want to have a
variable that contains a grade for a student, name it as, grade and not just some
random letters you choose.
3. Declare one variable per line of code. For example, the variable declarations,
double exam=0;
double quiz=10;
double grade = 0;
is preferred over the declaration,
double exam=0, quiz=10, grade=0;
Introduction to Programming I 67](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-67-320.jpg)
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4.9.2 Outputting Variable Data
In order to output the value of a certain variable, we can use the following commands,
System.out.println()
System.out.print()
Here's a sample program,
public class OutputVariable
{
public static void main( String[] args ){
int value = 10;
char x;
x = ‘A’;
System.out.println( value );
System.out.println( “The value of x=“ + x );
}
}
The program will output the following text on screen,
10
The value of x=A
4.9.3 System.out.println() vs. System.out.print()
What is the difference between the commands System.out.println() and
System.out.print()? The first one appends a newline at the end of the data to output,
while the latter doesn't.
Consider the statements,
System.out.print("Hello ");
System.out.print("world!");
These statements will output the following on the screen,
Hello world!
Now consider the following statements,
System.out.println("Hello ");
System.out.println("world!");
These statements will output the following on the screen,
Hello
world!
Introduction to Programming I 68](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-68-320.jpg)
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Here's a sample program in the usage of these operators:
public class ArithmeticDemo
{
public static void main(String[] args)
{
//a few numbers
int i = 37;
int j = 42;
double x = 27.475;
double y = 7.22;
System.out.println("Variable values...");
System.out.println(" i = " + i);
System.out.println(" j = " + j);
System.out.println(" x = " + x);
System.out.println(" y = " + y); //adding numbers
System.out.println("Adding...");
System.out.println(" i + j = " + (i + j));
System.out.println(" x + y = " + (x + y));
//subtracting numbers
System.out.println("Subtracting...");
System.out.println(" i - j = " + (i - j));
System.out.println(" x - y = " + (x - y));
//multiplying numbers
System.out.println("Multiplying...");
System.out.println(" i * j = " + (i * j));
System.out.println(" x * y = " + (x * y));
//dividing numbers
System.out.println("Dividing...");
System.out.println(" i / j = " + (i / j));
System.out.println(" x / y = " + (x / y));
//computing the remainder resulting from dividing
numbers
System.out.println("Computing the remainder...");
System.out.println(" i % j = " + (i % j));
System.out.println(" x % y = " + (x % y));
//mixing types
System.out.println("Mixing types...");
System.out.println(" j + y = " + (j + y));
System.out.println(" i * x = " + (i * x));
}
}
Introduction to Programming I 71](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-71-320.jpg)
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Here's a sample program that uses relational operators,
public class RelationalDemo
{
public static void main(String[] args) {
//a few numbers
int i = 37;
int j = 42;
int k = 42;
System.out.println("Variable values...");
System.out.println(" i = " + i);
System.out.println(" j = " + j);
System.out.println(" k = " + k);
//greater than
System.out.println("Greater than...");
System.out.println(" i > j = " + (i > j)); //false
System.out.println(" j > i = " + (j > i)); //true
System.out.println(" k > j = " + (k > j)); //false
//greater than or equal to
System.out.println("Greater than or equal to...");
System.out.println(" i >= j = " + (i >= j)); //false
System.out.println(" j >= i = " + (j >= i)); //true
System.out.println(" k >= j = " + (k >= j)); //true
//less than
System.out.println("Less than...");
System.out.println(" i < j = " + (i < j)); //true
System.out.println(" j < i = " + (j < i)); //false
System.out.println(" k < j = " + (k < j)); //false
//less than or equal to
System.out.println("Less than or equal to...");
System.out.println(" i <= j = " + (i <= j)); //true
System.out.println(" j <= i = " + (j <= i)); //false
System.out.println(" k <= j = " + (k <= j)); //true
//equal to
System.out.println("Equal to...");
System.out.println(" i == j = " + (i == j)); //false
System.out.println(" k == j = " + (k == j)); //true
//not equal to
System.out.println("Not equal to...");
System.out.println(" i != j = " + (i != j)); //true
System.out.println(" k != j = " + (k != j)); //false
}
}
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4.10.4.1 && (logical AND) and & (boolean logical AND)
Here is the truth table for && and &,
x1 x2 Result
TRUE TRUE TRUE
TRUE FALSE FALSE
FALSE TRUE FALSE
FALSE FALSE FALSE
Table 13: Truth table for & and &&
The basic difference between && and & operators is that && supports short-circuit
evaluations (or partial evaluations), while & doesn't. What does this mean?
Given an expression,
exp1 && exp2
&& will evaluate the expression exp1, and immediately return a false value is exp1 is
false. If exp1 is false, the operator never evaluates exp2 because the result of the
operator will be false regardless of the value of exp2. In contrast, the & operator always
evaluates both exp1 and exp2 before returning an answer.
Here's a sample source code that uses logical and boolean AND,
public class TestAND
{
public static void main( String[] args ){
int i = 0;
int j = 10;
boolean test= false;
//demonstrate &&
test = (i > 10) && (j++ > 9);
System.out.println(i);
System.out.println(j);
System.out.println(test);
//demonstrate &
test = (i > 10) & (j++ > 9);
System.out.println(i);
System.out.println(j);
System.out.println(test);
}
}
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4.10.4.2 || (logical OR) and | (boolean logical inclusive OR)
Here is the truth table for || and |,
x1 x2 Result
TRUE TRUE TRUE
TRUE FALSE TRUE
FALSE TRUE TRUE
FALSE FALSE FALSE
Table 14: Truth table for | and ||
The basic difference between || and | operators is that || supports short-circuit
evaluations (or partial evaluations), while | doesn't. What does this mean?
Given an expression,
exp1 || exp2
|| will evaluate the expression exp1, and immediately return a true value is exp1 is true.
If exp1 is true, the operator never evaluates exp2 because the result of the operator will
be true regardless of the value of exp2. In contrast, the | operator always evaluates both
exp1 and exp2 before returning an answer.
Here's a sample source code that uses logical and boolean OR,
public class TestOR
{
public static void main( String[] args ){
int i = 0;
int j = 10;
boolean test= false;
//demonstrate ||
test = (i < 10) || (j++ > 9);
System.out.println(i);
System.out.println(j);
System.out.println(test);
//demonstrate |
test = (i < 10) | (j++ > 9);
System.out.println(i);
System.out.println(j);
System.out.println(test);
}
}
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4.10.4.3 ^ (boolean logical exclusive OR)
Here is the truth table for ^,
x1 x2 Result
TRUE TRUE FALSE
TRUE FALSE TRUE
FALSE TRUE TRUE
FALSE FALSE FALSE
Table 15: Truth table for ^
The result of an exclusive OR operation is TRUE, if and only if one operand is true and
the other is false. Note that both operands must always be evaluated in order to
calculate the result of an exclusive OR.
Here's a sample source code that uses the logical exclusive OR operator,
public class TestXOR
{
public static void main( String[] args ){
boolean val1 = true;
boolean val2 = true;
System.out.println(val1 ^ val2);
val1 = false;
val2 = true;
System.out.println(val1 ^ val2);
val1 = false;
val2 = false;
System.out.println(val1 ^ val2);
val1 = true;
val2 = false;
System.out.println(val1 ^ val2);
}
}
The output of the program is,
false
true
false
true
Introduction to Programming I 83](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-83-320.jpg)
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4.10.4.4 ! (logical NOT)
The logical NOT takes in one argument, wherein that argument can be an expression,
variable or constant. Here is the truth table for !,
x1 Result
TRUE FALSE
FALSE TRUE
Table 16: Truth table for !
Here's a sample source code that uses the logical NOT operator,
public class TestNOT
{
public static void main( String[] args ){
boolean val1 = true;
boolean val2 = false;
System.out.println(!val1);
System.out.println(!val2);
}
}
The output of the program is,
false
true
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4.10.5 Conditional Operator (?:)
The conditional operator ?: is a ternary operator. This means that it takes in three
arguments that together form a conditional expression. The structure of an expression
using a conditional operator is,
exp1?exp2:exp3
wherein exp1 is a boolean expression whose result must either be true or false.
If exp1 is true, exp2 is the value returned. If it is false, then exp3 is returned.
For example, given the code,
public class ConditionalOperator
{
public static void main( String[] args ){
String status = "";
int grade = 80;
//get status of the student
status = (grade >= 60)?"Passed":"Fail";
//print status
System.out.println( status );
}
}
The output of this program will be,
Passed
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Here is the flowchart of how ?: works,
Here is another program that uses the ?: operator,
class ConditionalOperator
{
public static void main( String[] args ){
int score = 0;
char answer = 'a';
score = (answer == 'a') ? 10 : 0;
System.out.println("Score = " + score );
}
}
The output of the program is,
Score = 10
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Figure 4.2: Flowchart using the ?: operator](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-86-320.jpg)
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Here is the complete source code:
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
public class GetInputFromKeyboard
{
public static void main( String[] args ){
BufferedReader dataIn = new BufferedReader(new
InputStreamReader( System.in) );
String name = "";
System.out.print("Please Enter Your Name:");
try{
name = dataIn.readLine();
}catch( IOException e ){
System.out.println("Error!");
}
System.out.println("Hello " + name +"!");
}
}
Now let's try to explain each line of code:
The statements,
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
indicate that we want to use the classes BufferedReader, InputStreamReader and
IOException which is inside the java.io package. The Java Application Programming
Interface (API) contains hundreds of predefined classes that you can use in your
programs. These classes are organized into what we call packages.
Packages contain classes that have related purpose. Just like in our example, the
java.io package contains classes that allow programs to input and output data. The
statements can also be rewritten as,
import java.io.*;
which will load all the classes found in the package, and then we can use those classes
inside our program.
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The next two statements,
public class GetInputFromKeyboard
{
public static void main( String[] args ){
were already discussed in the previous lesson. This means we declare a class named
GetInputFromKeyboard and we declare the main method.
In the statement,
BufferedReader dataIn = new BufferedReader(new
InputStreamReader( System.in) );
we are declaring a variable named dataIn with the class type BufferedReader. Don't
worry about what the syntax means for now. We will cover more about this later in the
course.
Now, we are declaring a String variable with the identifier name,
String name = "";
This is where we will store the input of the user. The variable name is initialized to an
empty String "". It is always good to initialize your variables as you declare them.
The next line just outputs a String on the screen asking for the user's name.
System.out.print("Please Enter Your Name:");
Now, the following block defines a try-catch block,
try{
name = dataIn.readLine();
}catch( IOException e ){
System.out.println("Error!");
}
This assures that the possible exceptions that could occur in the statement
name = dataIn.readLine();
will be catched. We will cover more about exception handling in the latter part of this
course, but for now, just take note that you need to add this code in order to use the
readLine() method of BufferedReader to get input from the user.
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5.3 Using JOptionPane to get input
Another way to get input from the user is by using the JOptionPane class which is found
in the javax.swing package. JOptionPane makes it easy to pop up a standard dialog
box that prompts users for a value or informs them of something.
Given the following code,
import javax.swing.JOptionPane;
public class GetInputFromKeyboard
{
public static void main( String[] args ){
String name = "";
name = JoptionPane.showInputDialog("Please enter your
name");
String msg = "Hello " + name + "!";
JOptionPane.showMessageDialog(null, msg);
}
}
This will output,
Introduction to Programming I 93
Figure 5.1: Getting Input Using JOptionPane
Figure 5.2: Input florence on the JOptionPane
Figure 5.3: Showing Message Using JOptionPane](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-93-320.jpg)
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6.2.5 Example for if-else-else if
public class Grade
{
public static void main( String[] args )
{
double grade = 92.0;
if( grade >= 90 ){
System.out.println( "Excellent!" );
}
else if( (grade < 90) && (grade >= 80)){
System.out.println("Good job!" );
}
else if( (grade < 80) && (grade >= 60)){
System.out.println("Study harder!" );
}
else{
System.out.println("Sorry, you failed.");
}
}
}
Introduction to Programming I 102](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-102-320.jpg)
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6.2.7 Example for switch
public class Grade
{
public static void main( String[] args )
{
int grade = 92;
switch(grade){
case 100:
System.out.println( "Excellent!" );
break;
case 90:
System.out.println("Good job!" );
break;
case 80:
System.out.println("Study harder!" );
break;
default:
System.out.println("Sorry, you failed.");
}
}
}
Introduction to Programming I 105](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-105-320.jpg)
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6.4 Branching Statements
Branching statements allows us to redirect the flow of program execution. Java offers
three branching statements: break, continue and return.
6.4.1 break statement
The break statement has two forms: unlabeled (we saw its unlabeled form in the switch
statement) and labeled.
6.4.1.1 Unlabeled break statement
The unlabeled break terminates the enclosing switch statement, and flow of control
transfers to the statement immediately following the switch. You can also use the
unlabeled form of the break statement to terminate a for, while, or do-while loop.
For example,
String names[] = {"Beah", "Bianca", "Lance", "Belle",
"Nico", "Yza", "Gem", "Ethan"};
String searchName = "Yza";
boolean foundName = false;
for( int i=0; i< names.length; i++ ){
if( names[i].equals( searchName )){
foundName = true;
break;
}
}
if( foundName ){
System.out.println( searchName + " found!" );
}
else{
System.out.println( searchName + " not found." );
}
In this example, if the search string "Yza" is found, the for loop will stop and flow of
control transfers to the statement following the for loop.
Introduction to Programming I 110](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-110-320.jpg)
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6.4.1.2 Labeled break statement
The labeled form of a break statement terminates an outer statement, which is identified
by the label specified in the break statement. The following program searches for a value
in a two-dimensional array. Two nested for loops traverse the array. When the value is
found, a labeled break terminates the statement labeled search, which is the outer for
loop.
int[][] numbers = {{1, 2, 3}, {4, 5, 6},
{7, 8, 9}};
int searchNum = 5;
boolean foundNum = false;
searchLabel:
for( int i=0; i<numbers.length; i++ ){
for( int j=0; j<numbers[i].length; j++ ){
if( searchNum == numbers[i][j] ){
foundNum = true;
break searchLabel;
}
}
}
if( foundNum ){
System.out.println( searchNum + " found!" );
}
else{
System.out.println( searchNum + " not found!" );
}
The break statement terminates the labeled statement; it does not transfer the flow of
control to the label. The flow of control transfers to the statement immediately following
the labeled (terminated) statement.
Introduction to Programming I 111](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-111-320.jpg)
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6.4.2 continue statement
The continue statement has two forms: unlabeled and labeled. You can use the continue
statement to skip the current iteration of a for, while or do-while loop.
6.4.2.1 Unlabeled continue statement
The unlabeled form skips to the end of the innermost loop's body and evaluates the
boolean expression that controls the loop, basically skipping the remainder of this
iteration of the loop.
The following example counts the number of "Beah"s in the array.
String names[] = {"Beah", "Bianca", "Lance", "Beah"};
int count = 0;
for( int i=0; i<names.length; i++ ){
if( !names[i].equals("Beah") ){
continue; //skip next statement
}
count++;
}
System.out.println("There are " + count + " Beahs in the
list");
6.4.2.2 Labeled continue statement
The labeled form of the continue statement skips the current iteration of an outer loop
marked with the given label.
outerLoop:
for( int i=0; i<5; i++ ){
for( int j=0; j<5; j++ ){
System.out.println("Inside for(j) loop"); //message1
if( j == 2 ) continue outerLoop;
}
System.out.println("Inside for(i) loop"); //message2
}
In this example, message 2 never gets printed since we have the statement continue
outerloop which skips the iteration.
Introduction to Programming I 112](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-112-320.jpg)
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7.3 Declaring Arrays
Arrays must be declared like all variables. When declaring an array, you list the data
type, followed by a set of square brackets[], followed by the identifier name. For
example,
int []ages;
or you can place the brackets after the identifier. For example,
int ages[];
After declaring, we must create the array and specify its length with a constructor
statement. This process in Java is called instantiation (the Java word for creates). In
order to instantiate an object, we need to use a constructor for this. We will cover more
about instantiating objects and constructors later. Take note, that the size of an array
cannot be changed once you've initialized it. For example,
//declaration
int ages[];
//instantiate object
ages = new int[100];
or, can also be written as,
//declare and instantiate
object
int ages[] = new
int[100];
In the example, the declaration tells the
Java Compiler that the identifier ages will
be used as the name of an array containing
integers, and to create or instantiate a new
array containing 100 elements.
Instead of using the new keyword to
instantiate an array, you can also
automatically declare, construct and assign
values at once.
Introduction to Programming I 116
Figure 7.2: Instantiating Arrays](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-116-320.jpg)
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Examples are,
//creates an array of boolean variables with ientifier
//results. This array contains 4 elements that are
//initialized to values {true, false, true, false}
boolean results[] ={ true, false, true, false };
//creates an array of 4 double variables initialized
//to the values {100, 90, 80, 75};
double []grades = {100, 90, 80, 75};
//creates an array of Strings with identifier days and
//initialized. This array contains 7 elements
String days[] = { “Mon”, “Tue”, “Wed”, “Thu”, “Fri”, “Sat”, “Sun”};
Introduction to Programming I 117](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-117-320.jpg)
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7.4 Accessing an array element
To access an array element, or a part of the array, you use a number called an index or
a subscript.
An index number or subscript is assigned to each member of the array, allowing the
program and the programmer to access individual values when necessary. Index
numbers are always integers. They begin with zero and progress sequentially by
whole numbers to the end of the array. Take note that the elements inside your
array is from 0 to (sizeOfArray-1).
For example, given the array we declared a while ago, we have
//assigns 10 to the first element in the array
ages[0] = 10;
//prints the last element in the array
System.out.print(ages[99]);
Take note that once an array is declared and constructed, the stored value of each
member of the array will be initialized to zero for number data. However, reference data
types such as Strings are not initialized to blanks or an empty string “”. Therefore, you
must populate the String arrays explicitly.
The following is a sample code on how to print all the elements in the array. This uses a
for loop, so our code is shorter.
public class ArraySample{
public static void main( String[] args ){
int[] ages = new int[100];
for( int i=0; i<100; i++ ){
System.out.print( ages[i] );
}
}
}
Coding Guidelines:
1. It is usually better to initialize or instantiate the array right away after you declare it.
For example, the declaration,
int []arr = new int[100];
is preferred over,
int []arr;
arr = new int[100];
2. The elements of an n-element array have indexes from 0 to n-1. Note that there is no
array element arr[n]! This will result in an array-index-out-of-bounds exception.
3. You cannot resize an array.
Introduction to Programming I 118](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-118-320.jpg)
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7.5 Array length
In order to get the number of elements in an array, you can use the length field of an
array. The length field of an array returns the size of the array. It can be used by writing,
arrayName.length
For example, given the previous example, we can re-write it as,
public class ArraySample
{
public static void main( String[] args ){
int[] ages = new int[100];
for( int i=0; i<ages.length; i++ ){
System.out.print( ages[i] );
}
}
}
Coding Guidelines:
1. When creating for loops to process the elements of an array, use the array object's
length field in the condition statement of the for loop. This will allow the loop to
adjust automatically for different-sized arrays.
2. Declare the sizes of arrays in a Java program using named constants to make them
easy to change. For example,
final int ARRAY_SIZE = 1000; //declare a constant
. . .
int[] ages = new int[ARRAY_SIZE];
Introduction to Programming I 119](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-119-320.jpg)
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7.6 Multidimensional Arrays
Multidimensional arrays are implemented as arrays of arrays. Multidimensional arrays
are declared by appending the appropriate number of bracket pairs after the array name.
For example,
// integer array 512 x 128 elements
int[][] twoD = new int[512][128];
// character array 8 x 16 x 24
char[][][] threeD = new char[8][16][24];
// String array 4 rows x 2 columns
String[][] dogs = {{ "terry", "brown" },
{ "Kristin", "white" },
{ "toby", "gray"},
{ "fido", "black"}
};
To access an element in a multidimensional array is just the same as accessing the
elements in a one dimensional array. For example, to access the first element in the first
row of the array dogs, we write,
System.out.print( dogs[0][0] );
This will print the String "terry" on the screen.
Introduction to Programming I 120](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-120-320.jpg)
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7.7 Exercises
7.7.1 Days of the Week
Create an array of Strings which are initialized to the 7 days of the week. For Example,
String days[] = {“Monday”, “Tuesday”….};
Using a while-loop, print all the contents of the array. (do the same for do-while and for-
loop)
7.7.2 Greatest number
Using BufferedReader or JOptionPane, ask for 10 numbers from the user. Use an array to
store the values of these 10 numbers. Output on the screen the number with the
greatest value.
7.7.3 Addressbook Entries
Given the following multidimensional array that contains addressbook entries:
String entry = {{"Florence", "735-1234", "Manila"},
{"Joyce", "983-3333", "Quezon City"},
{"Becca", "456-3322", "Manila"}};
Print the following entries on screen in the following format:
Name : Florence
Tel. # : 735-1234
Address : Manila
Name : Joyce
Tel. # : 983-3333
Address : Quezon City
Name : Becca
Tel. # : 456-3322
Address : Manila
Introduction to Programming I 121](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-121-320.jpg)
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In the Java language, when you invoke an application, the runtime system passes the
command-line arguments to the application's main method via an array of Strings. Each
String in the array contains one of the command-line arguments. Remember the
declaration for the main method,
public static void main( String[] args )
The arguments that are passed to your program are saved into an array of String with
the args identifier.
In the previous example, the command-line arguments passed to the Sort application is
an array that contains five strings which are: "5", "4", "3", "2" and "1". You can derive
the number of command-line arguments with the array's length attribute.
For example,
int numberOfArgs = args.length;
If your program needs to support a numeric command-line argument, it must convert a
String argument that represents a number, such as "34", to a number. Here's a code
snippet that converts a command-line argument to an integer,
int firstArg = 0;
if (args.length > 0){
firstArg = Integer.parseInt(args[0]);
}
parseInt throws a NumberFormatException (ERROR) if the format of args[0] isn't valid
(not a number).
Coding Guidelines:
Before using command-line arguments, always check if the number of arguments before
accessing the array elements so that there will be no exception generated.
Introduction to Programming I 123](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-123-320.jpg)
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8.3 Command-line arguments in NetBeans
To illustrate on how to pass some arguments to your programs in NetBeans, let us create
a Java program that will print the number of arguments and the first argument passed to
it.
public class CommandLineExample
{
public static void main( String[] args ){
System.out.println("Number of arguments=" +
args.length);
System.out.println("First Argument="+ args[0]);
}
}
Now, run netbeans and create a new project and name this CommandLineExample. Copy
the code shown above and compile the code. Now, follow these steps to pass arguments
to your program using NetBeans.
Click on Projects (encircled below).
Introduction to Programming I 124
Figure 8.2: Opening Project File](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-124-320.jpg)
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9.4.3 Passing Variables in Methods
In our examples, we already tried passing variables to methods. However, we haven't
differentiated between the different types of variable passing in Java. There are two
types of passing data to methods, the first one is pass-by-value and then, pass-by-
reference.
9.4.3.1 Pass-by-value
When a pass-by-value occurs, the method makes a copy of the value of the variable
passed to the method. The method cannot accidentally modify the original argument
even if it modifies the parameters during calculations.
For example,
public class TestPassByValue
{
public static void main( String[] args ){
int i = 10;
//print the value of i
System.out.println( i );
//call method test
//and pass i to method test
test( i );
//print the value of i. i not changed
System.out.println( i );
}
public static void test( int j ){
//change value of parameter j
j = 33;
}
}
In the given example, we called the method test and passed the value of i as parameter.
The value of i is copied to the variable of the method j. Since j is the variable changed in
the test method, it will not affect the variable value if i in main since it is a different copy
of the variable.
By default, all primitive data types when passed to a method are pass-by-value.
Introduction to Programming I 135
Pass i as parameter
which is copied to j](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-135-320.jpg)
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9.4.3.2 Pass-by-reference
When a pass-by-reference occurs, the reference to an object is passed to the calling
method. This means that, the method makes a copy of the reference of the variable
passed to the method. However, unlike in pass-by-value, the method can modify the
actual object that the reference is pointing to, since, although different references are
used in the methods, the location of the data they are pointing to is the same.
For example,
class TestPassByReference
{
public static void main( String[] args ){
//create an array of integers
int []ages = {10, 11, 12};
//print array values
for( int i=0; i<ages.length; i++ ){
System.out.println( ages[i] );
}
//call test and pass reference to array
test( ages );
//print array values again
for( int i=0; i<ages.length; i++ ){
System.out.println( ages[i] );
}
}
public static void test( int[] arr ){
//change values of array
for( int i=0; i<arr.length; i++ ){
arr[i] = i + 50;
}
}
}
Introduction to Programming I 136
Pass ages as parameter
which is copied to
variable arr](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-136-320.jpg)
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9.4.5 Scope of a variable
In addition to a variable's data type and name, a variable has scope. The scope
determines where in the program the variable is accessible. The scope also determines
the lifetime of a variable or how long the variable can exist in memory. The scope is
determined by where the variable declaration is placed in the program.
To simplify things, just think of the scope as anything between the curly braces {...}.
The outer curly braces is called the outer blocks, and the inner curly braces is called
inner blocks.
If you declare variables in the outer block, they are visible (i.e. usable) by the program
lines inside the inner blocks. However, if you declare variables in the inner block, you
cannot expect the outer block to see it.
A variable's scope is inside the block where it is declared, starting from the point where it
is declared, and in the inner blocks.
For example, given the following code snippet,
public class ScopeExample
{
public static void main( String[] args ){
int i = 0;
int j = 0;
//... some code here
{
int k = 0;
int m = 0;
int n = 0;
}
}
The code we have here represents five scopes indicated by the lines and the letters
representing the scope. Given the variables i,j,k,m and n, and the five scopes A,B,C,D
and E, we have the following scopes for each variable:
The scope of variable i is A.
The scope of variable j is B.
The scope of variable k is C.
The scope of variable m is D.
The scope of variable n is E.
Introduction to Programming I 138
A
E
B
D
C](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-138-320.jpg)
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Now, given the two methods main and test in our previous examples,
class TestPassByReference
{
public static void main( String[] args ){
//create an array of integers
int []ages = {10, 11, 12};
//print array values
for( int i=0; i<ages.length; i++ ){
System.out.println( ages[i] );
}
//call test and pass reference to array
test( ages );
//print array values again
for( int i=0; i<ages.length; i++ ){
System.out.println( ages[i] );
}
}
public static void test( int[] arr ){
//change values of array
for( int i=0; i<arr.length; i++ ){
arr[i] = i + 50;
}
}
}
In the main method, the scope of the variables are,
ages[] - scope A
i in B - scope B
i in C – scope C
In the test method, the scope ofthe variables are,
arr[] - scope D
i in E - scope E
Introduction to Programming I 139
A
B
C
E D](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-139-320.jpg)
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9.5.4 Comparing Objects
In our previous discussions, we learned about operators for comparing values—equal,
not equal, less than, and so on. Most of these operators work only on primitive types, not
on objects. If you try to use other values as operands, the Java compiler produces errors.
The exceptions to this rule are the operators for equality: == (equal) and != (not equal).
When applied to objects, these operators don't do what you might first expect. Instead of
checking whether one object has the same value as the other object, they determine
whether both sides of the operator refer to the same object.
To compare instances of a class and have meaningful results, you must implement
special methods in your class and call those methods. A good example of this is the
String class.
It is possible to have two different String objects that contain the same values. If you
were to employ the == operator to compare these objects, however, they would be
considered unequal. Although their contents match, they are not the same object.
To see whether two String objects have matching values, a method of the class called
equals() is used. The method tests each character in the string and returns true if the
two strings have the same values.
The following code illustrates this,
class EqualsTest {
public static void main(String[] arguments) {
String str1, str2;
str1 = "Free the bound periodicals.";
str2 = str1;
System.out.println("String1: " + str1);
System.out.println("String2: " + str2);
System.out.println("Same object? " + (str1 == str2));
str2 = new String(str1);
System.out.println("String1: " + str1);
System.out.println("String2: " + str2);
System.out.println("Same object? " + (str1 == str2));
System.out.println("Same value? " + str1.equals(str2));
}
}
This program's output is as follows,
OUTPUT:
String1: Free the bound periodicals.
String2: Free the bound periodicals.
Same object? true
String1: Free the bound periodicals.
String2: Free the bound periodicals.
Same object? false
Same value? True
Introduction to Programming I 146](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-146-320.jpg)
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10 Creating your own Classes
10.1 Objectives
Now that we've studied on how to use existing classes from the Java class library, we will
now be studying on how to write our own classes. For this section, in order to easily
understand how to create classes, we will make a sample class wherein we will add more
data and functionality as we go along the way.
We will create a class that contains information of a Student and operations needed for a
certain student record.
Things to take note of for the syntax defined in this section and for the other sections:
* - means that there may be 0 or more occurrences of the line
whereit was applied to.
<description> - indicates that you have to substitute an actual value for this part
instead of typing it as it is.
[] - indicates that this part is optional
At the end of the lesson, the student should be able to:
• Create their own classes
• Declare attributes and methods for their classes
• Use the this reference to access instance data
• Create and call overloaded methods
• Import and create packages
• Use access modifiers to control access to class members
Introduction to Programming I 150](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-150-320.jpg)
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10.3 Declaring Attributes
To declare a certain attribute for our class, we write,
<modifier> <type> <name> [= <default_value>];
Now, let us write down the list of attributes that a student record can contain. For each
information, also list what data types would be appropriate to use. For example, you
don't want to have a data type int for a student's name, or a String for a student's grade.
The following are some sample information we want to add to the student record.
name - String
address - String
age - int
math grade - double
english grade - double
science grade - double
average grade - double
You can add more information if you want to, it's all really up to you. But for this
example, we will be using these information.
10.3.1 Instance Variables
Now that we have a list of all the attributes we want to add to our class, let us now add
them to our code. Since we want these attributes to be unique for each object (or for
each student), we should declare them as instance variables.
For example,
public class StudentRecord
{
private String name;
private String address;
private int age;
private double mathGrade;
private double englishGrade;
private double scienceGrade;
private double average;
//we'll add more code here later
}
where,
private here means that the variables are only accessible within the class. Other objects
cannot access these variables directly. We will cover more about accessibility later.
Coding Guidelines:
1. Declare all your instance variables on the top of the class declaration.
2. Declare one variable for each line.
3. Instance variables, like any other variables should start with a SMALL letter.
4. Use an appropriate data type for each variable you declare.
5. Declare instance variables as private so that only class methods can access them
directly.
Introduction to Programming I 152](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-152-320.jpg)
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10.3.2 Class Variables or Static Variables
Aside from instance variables, we can also declare class variables or variables that
belong to the class as a whole. The value of these variables are the same for all the
objects of the same class. Now suppose, we want to know the total number of student
records we have for the whole class, we can declare one static variable that will hold this
value. Let us call this as studentCount.
To declare a static variable,
public class StudentRecord
{
//instance variables we have declared
private static int studentCount;
//we'll add more code here later
}
we use the keyword static to indicate that a variable is a static variable.
So far, our whole code now looks like this.
public class StudentRecord
{
private String name;
private String address;
private int age;
private double mathGrade;
private double englishGrade;
private double scienceGrade;
private double average;
private static int studentCount;
//we'll add more code here later
}
10.4 Declaring Methods
Before we discuss what methods we want our class to have, let us first take a look at the
general syntax for declaring methods.
To declare methods we write,
<modifier> <returnType> <name>(<parameter>*) {
<statement>*
}
where,
<modifier> can carry a number of different modifiers
<returnType> can be any data type (including void)
<name> can be any valid identifier
<parameter> ::= <parameter_type> <parameter_name>[,]
Introduction to Programming I 153](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-153-320.jpg)
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10.4.3 Multiple Return statements
You can have multiple return statements for a method as long as they are not on the
same block. You can also use constants to return values instead of variables.
For example, consider the method,
public String getNumberInWords( int num ){
String defaultNum = "zero";
if( num == 1 ){
return "one"; //return a constant
}
else if( num == 2){
return "two"; //return a constant
}
//return a variable
return defaultNum;
}
10.4.4 Static methods
For the static variable studentCount, we can create a static method to access its value.
public class StudentRecord
{
private static int studentCount;
public static int getStudentCount(){
return studentCount;
}
}
where,
public - means that the method can be called from objects outside the
class
static - means that the method is static and should be called by
typing,[ClassName].[methodName]. For example, in this case,
we call the method StudentRecord.getStudentCount()
int - is the return type of the method. This means that the method
should return a value of type int
getStudentCount - the name of the method
() - this means that our method does not have any parameters
For now, getStudentCount will always return the value zero since we haven't done
anything yet in our program in order to set its value. We will try to change the value of
studentCount later on when we discuss constructors.
Coding Guidelines:
1. Method names should start with a SMALL letter.
2. Method names should be verbs
3. Always provide documentation before the declaration of the method. You can use
javadocs style for this. Please see example.
Introduction to Programming I 156](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-156-320.jpg)
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Now, here's a sample code of a class that uses our StudentRecord class.
public class StudentRecordExample
{
public static void main( String[] args ){
//create three objects for Student record
StudentRecord annaRecord = new StudentRecord();
StudentRecord beahRecord = new StudentRecord();
StudentRecord crisRecord = new StudentRecord();
//set the name of the students
annaRecord.setName("Anna");
beahRecord.setName("Beah");
crisRecord.setName("Cris");
//print anna's name
System.out.println( annaRecord.getName() );
//print number of students
System.out.println("Count="+StudentRecord.getStudentCount());
}
}
The output of this program is,
Anna
Student Count = 0
Introduction to Programming I 158](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-158-320.jpg)
![J.E.D.I
When we try to call this in the following main method,
public static void main( String[] args )
{
StudentRecord annaRecord = new StudentRecord();
annaRecord.setName("Anna");
annaRecord.setAddress("Philippines");
annaRecord.setAge(15);
annaRecord.setMathGrade(80);
annaRecord.setEnglishGrade(95.5);
annaRecord.setScienceGrade(100);
//overloaded methods
annaRecord.print( annaRecord.getName() );
annaRecord.print( annaRecord.getEnglishGrade(),
annaRecord.getMathGrade(),
annaRecord.getScienceGrade());
}
we will have the output for the first call to print,
Name:Anna
Address:Philippines
Age:15
we will have the output for the second call to print,
Name:Anna
Math Grade:80.0
English Grade:95.5
Science Grade:100.0
Always remember that overloaded methods have the following properties,
– the same name
– different parameters
– return types can be different or the same
Introduction to Programming I 161](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-161-320.jpg)
![J.E.D.I
10.7.3 Using Constructors
To use these constructors, we have the following code,
public static void main( String[] args )
{
//create three objects for Student record
StudentRecord annaRecord=new StudentRecord("Anna");
StudentRecord beahRecord=new StudentRecord("Beah",
"Philippines");
StudentRecord crisRecord=new
StudentRecord(80,90,100);
//some code here
}
Now, before we move on, let us go back to the static variable studentCount we have
declared a while ago. The purpose of the studentCount is to count the number of objects
that are instantiated with the class StudentRecord. So, what we want to do here is,
everytime an object of class StudentRecord is instantiated, we increment the value of
studentCount. A good location to modify and increment the value of studentCount is in
the constructors, because it is always called everytime an object is instantiated. For
example,
public StudentRecord(){
//some initialization code here
studentCount++; //add a student
}
public StudentRecord(String temp){
this.name = temp;
studentCount++; //add a student
}
public StudentRecord(String name, String address){
this.name = name;
this.address = address;
studentCount++; //add a student
}
public StudentRecord(double mGrade, double eGrade,
double sGrade){
mathGrade = mGrade;
englishGrade = eGrade;
scienceGrade = sGrade;
studentCount++; //add a student
}
Introduction to Programming I 163](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-163-320.jpg)
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10.7.4 The this() Constructor Call
Constructor calls can be chained, meaning, you can call another constructor from inside
another constructor. We use the this() call for this. For example, given the following
code,
1: public StudentRecord(){
2: this("some string");
3:
4: }
5:
6: public StudentRecord(String temp){
7: this.name = temp;
8: }
9:
10: public static void main( String[] args )
11: {
12:
13: StudentRecord annaRecord = new StudentRecord();
14: }
Given the code above, when the statement at line 13 is called, it will call the default
constructor line 1. When statement in line 2 is executed, it will then call the constructor
that has a String parameter (in line 6).
There are a few things to remember when using the this constructor call:
1. When using the this constructor call, IT MUST OCCUR AS THE FIRST STATEMENT
in a constructor
2. It can ONLY BE USED IN A CONSTRUCTOR DEFINITION. The this call can then be
followed by any other relevant statements.
Introduction to Programming I 164](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-164-320.jpg)
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Now, we want to create another class named Student. Since a student is also a person,
we decide to just extend the class Person, so that we can inherit all the properties and
methods of the existing class Person. To do this, we write,
public class Student extends Person
{
public Student(){
System.out.println(“Inside Student:Constructor”);
//some code here
}
// some code here
}
When a Student object is instantiated, the default constructor of its superclass is invoked
implicitly to do the necessary initializations. After that, the statements inside the
subclass are executed. To illustrate this, consider the following code,
public static void main( String[] args ){
Student anna = new Student();
}
In the code, we create an object of class Student. The output of the program is,
Inside Person:Constructor
Inside Student:Constructor
The program flow is shown below.
Introduction to Programming I 173
Figure 11.2: Program Flow](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-173-320.jpg)
![J.E.D.I
11.3 Polymorphism
Now, given the parent class Person and the subclass Student of our previous example,
we add another subclass of Person which is Employee. Below is the class hierarchy for
that,
In Java, we can create a reference that is of type superclass to an object of its subclass.
For example,
public static main( String[] args )
{
Person ref;
Student studentObject = new Student();
Employee employeeObject = new Employee();
ref = studentObject; //Person ref points to a
// Student object
//some code here
}
Now suppose we have a getName method in our superclass Person, and we override this
method in both the subclasses Student and Employee,
public class Person
{
public String getName(){
System.out.println(“Person Name:” + name);
return name;
}
}
public class Student extends Person
{
public String getName(){
System.out.println(“Student Name:” + name);
return name;
}
}
public class Employee extends Person
{
public String getName(){
System.out.println(“Employee Name:” + name);
return name;
}
}
Introduction to Programming I 177
Figure 11.3: Hierarchy for Person class and it's
classes
Person
Student Employee](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-177-320.jpg)
![J.E.D.I
Going back to our main method, when we try to call the getName method of the
reference Person ref, the getName method of the Student object will be called. Now, if
we assign ref to an Employee object, the getName method of Employee will be called.
public static main( String[] args )
{
Person ref;
Student studentObject = new Student();
Employee employeeObject = new Employee();
ref = studentObject; //Person reference points to a
// Student object
String temp = ref.getName(); //getName of Student
//class is called
System.out.println( temp );
ref = employeeObject; //Person reference points to an
// Employee object
String temp = ref.getName(); //getName of Employee
//class is called
System.out.println( temp );
}
This ability of our reference to change behavior according to what object it is holding is
called polymorphism. Polymorphism allows multiple objects of different subclasses to
be treated as objects of a single superclass, while automatically selecting the proper
methods to apply to a particular object based on the subclass it belongs to.
Another example that exhibits the property of polymorphism is when we try to pass a
reference to methods. Suppose we have a static method printInformation that takes in
a Person object as reference, we can actually pass a reference of type Employee and type
Student to this method as long as it is a subclass of the class Person.
public static main( String[] args )
{
Student studentObject = new Student();
Employee employeeObject = new Employee();
printInformation( studentObject );
printInformation( employeeObject );
}
public static printInformation( Person p ){
. . . .
}
Introduction to Programming I 178](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-178-320.jpg)
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11.5.3 Interface vs. Class
One common characteristic of an interface and class is that they are both types. This
means that an interface can be used in places where a class can be used. For example,
given a class Person and an interface PersonInterface, the following declarations are
valid:
PersonInterface pi = new Person();
Person pc = new Person();
However, you cannot create an instance from an interface. An example of this is:
PersonInterface pi = new PersonInterface(); //COMPILE
//ERROR!!!
Another common characteristic is that both interface and class can define methods.
However, an interface does not have an implementation code while the class have one.
11.5.4 Creating Interfaces
To create an interface, we write,
public interface [InterfaceName]
{
//some methods without the body
}
As an example, let's create an interface that defines relationships between two objects
according to the “natural order” of the objects.
public interface Relation
{
public boolean isGreater( Object a, Object b);
public boolean isLess( Object a, Object b);
public boolean isEqual( Object a, Object b);
}
Now, to use the interface, we use the implements keyword. For example,
/**
* This class defines a line segment
*/
public class Line implements Relation
{
private double x1;
private double x2;
private double y1;
private double y2;
public Line(double x1, double x2, double y1, double y2){
this.x1 = x1;
this.x2 = x2;
this.y1 = y1;
this.y2 = y2;
}
public double getLength(){
Introduction to Programming I 182](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-182-320.jpg)
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Exceptions thrown during execution of the try block can be caught and handled in a
catch block. The code in the finally block is always executed.
The following are the key aspects about the syntax of the try-catch-finally construct:
• The block notation is mandatory.
• For each try block, there can be one or more catch blocks, but only one finally block.
• The catch blocks and finally blocks must always appear in conjunction with the try
block, and in the above order.
• A try block must be followed by at least one catch block OR one finally block, or both.
• Each catch block defines an exception handle. The header of the catch block takes
exactly one argument, which is the exception its block is willing to handle. The
exception must be of the Throwable class or one of its subclasses.
Let's take for example a code that prints the second argument when we try to run the
code using command-line arguments. Suppose, there is no checking inside your code for
the number of arguments and we just access the second argument args[1] right away,
we'll get the following exception.
Exception in thread "main"
java.lang.ArrayIndexOutOfBoundsException: 1
at ExceptionExample.main(ExceptionExample.java:5)
Introduction to Programming I 187
Figure 12.1: Flow of events in a try-catch-finally block](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-187-320.jpg)
![J.E.D.I
To prevent this from happening, we can place the code inside a try-catch block. The
finally block is just optional. For this example, we won't use the finally block.
public class ExceptionExample
{
public static void main( String[] args ){
try{
System.out.println( args[1] );
}catch( ArrayIndexOutOfBoundsException exp ){
System.out.println("Exception caught!");
}
}
}
So when we try to run the program again without arguments, the output would be,
Exception caught!
Introduction to Programming I 188](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-188-320.jpg)
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12.4 Exercises
12.4.1 Catching Exceptions1
Given the following code:
public class TestExceptions{
public static void main( String[] args ){
for( int i=0; true; i++ ){
System.out.println("args["+i+"]="+
args[i]);
}
}
}
Compile and run the TestExceptions program. The output should look like this:
javac TestExceptions one two three
args[0]=one
args[1]=two
args[2]=three
Exception in thread "main"
java.lang.ArrayIndexOutOfBoundsException: 3
at TestExceptions.main(1.java:4)
Modify the TestExceptions program to handle the exception. The output of the program
after catching the exception should look like this:
javac TestExceptions one two three
args[0]=one
args[1]=two
args[2]=three
Exception caught:
java.lang.ArrayIndexOutOfBoundsException: 3
Quiting...
12.4.2 Catching Exceptions 2
Chances are very good that some programs you've written before have encountered
exceptions. Since you didn't catch the exceptions, they simply halted the execution of
your code. Go back to those programs and implement exception handling.
Introduction to Programming I 189](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-189-320.jpg)
![J.E.D.I
Just press enter, until you see the question: Do you agree to the above license terms?
[yes or no]. Just type: yes and press ENTER. Just wait for the installer to finish
unpacking all its contents and installing Java.
Introduction to Programming I 193](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-193-320.jpg)
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Appendix B: Getting to know your Programming
Environment (Windows XP version)
In this section, we will be discussing on how to write, compile and run Java programs.
There are two ways of doing this, the first one is by using a console and a text editor.
The second one is by using NetBeans which is an Integrated Development
Environment or IDE.
An IDE is a programming environment integrated into a software application that
provides a GUI builder, a text or code editor, a compiler and/or interpreter and a
debugger.
Before going into details, let us first take a look at the first Java program you will be
writing.
My First Java Program
public class Hello
{
/**
* My first java program
*/
public static void main(String[] args) {
//prints the string "Hello world" on screen
System.out.println("Hello world!");
}
}
Before we try to explain what the program means, let's first try to write this program in a
file and try to run it.
Introduction to Programming I 215](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-215-320.jpg)
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To go inside a directory, you type in the command: cd [directory name]. The "cd"
command stands for, change directory. In this case, since the name of our directory is
My Documents, you type in: cd My Documents.
Now that you are inside the "My Documents" folder, try typing in the "dir" command
again, and tell me what you see.
Now perform the same steps described before to go inside the MYJAVAPROGRAMS folder.
Introduction to Programming I 226
Figure 12.21: Inside the My Documents folder
Figure 12.22: The contents of My Documents](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-226-320.jpg)
![J.E.D.I
Once inside the folder where your Java programs are, let us now start compiling your
Java program. Take note that, you should make sure that the file is inside the folder
where you are in. In order to do that, execute the dir command again to see if your file
is inside that folder.
To compile a Java program, we type in the command: javac [filename]. So in this
case, type in: javac Hello.java.
During compilation, javac adds a file to the disk called [filename].class, or in this case,
Hello.class, which is the actual bytecode.
Introduction to Programming I 227
Figure 12.23: Inside the MYJAVAPROGRAMS folder
Figure 12.24: Compile program by usingthe javac command](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-227-320.jpg)
![J.E.D.I
Step 6: Running the Program
Now, assuming that there are no problems during compilation (we'll explore more of the
problems encountered during compilation in the next section), we are now ready to run
your program.
To run your Java program, type in the command: java [filename without the
extension], so in the case of our example, type in: java Hello
You can see on the screen that you have just run your first Java program that prints the
message, "Hello world!".
Introduction to Programming I 228
Figure 12.25: Output of the program](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-228-320.jpg)
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Machine Problem 2: Minesweeper
This is a one player game of a simplified version of the popular computer game
minesweeper. First, the user is asked if he or she wants to play on a 5x5 grid or 10x10
grid. You have two 2-dimensional arrays that contains information about your grid. An
entry in the array should either contain a 0 or 1. A 1 signifies that there is a bomb in
that location, and a 0 if none.
For example, given the array:
int bombList5by5[][]={{0, 0, 1, 0, 0},
{0, 0, 0, 0, 0},
{0, 1, 0, 0, 0},
{0, 0, 0, 1, 1},
{0, 1, 1, 0, 0}};
Given the bomb list, we have 6 bombs on our list. The bombs are located in (row,col)
cells, (0,2), (2,1), (3,3), (3,4), (4,1) and (4,2).
If the user chooses a cell that contains a bomb, the game ends and all the bombs are
displayed. If the user chooses a cell that does not contain a bomb, a number appears at
that location indicating the number of neighbors that contain bombs. The game should
end when all the cells that do not contain bombs have been marked (player wins) or
when the user steps on a bomb(player loses).
Here's a sample output of the game, given the bombList5by5.
Welcome to Minesweeper!
Choose size of grid (Press 1 for 5x5, Press 2 for 10x10): 1
[ ] [ ] [ ] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
Enter row and column of the cell you want to open[row col]: 1 1
[ ] [ ] [ ] [ ] [ ]
[ ] [2] [ ] [ ] [ ]
[ ] [ ] [] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
Enter row and column of the cell you want to open[row col]: 3 2
[ ] [ ] [ ] [ ] [ ]
[ ] [2 ] [ ] [] [ ]
[ ] [ ] [ ] [ ] [ ]
[ ] [ ] [4 ] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
Enter row and column of the cell you want to open[row col]: 0 2
[] [ ] [ ] [ ] [ ]
[ ] [2] [ ] [] [ ]
[ ] [X ] [ ] [ ] [ ]
[ ] [ ] [4] [ ] [ ]
[ ] [ ] [ ] [ ] [ ]
Ooppps! You stepped on a bomb. Sorry, game over!
Introduction to Programming I 241](https://image.slidesharecdn.com/jedicoursenotes-introtoprogramming1-101028041314-phpapp01/85/Jedi-course-notes-intro-to-programming-1-241-320.jpg)
Jedi course notes intro to programming 1
- 1. J.E.D.I Introduction to Programming I Student's Manual Version 1.3 June 2006 Introduction to Programming I 1
- 2. J.E.D.I Author Florence Tiu Balagtas Team Joyce Avestro Florence Balagtas Rommel Feria Reginald Hutcherson Rebecca Ong John Paul Petines Sang Shin Raghavan Srinivas Matthew Thompson Requirements For the Laboratory Exercises Supported Operating Systems The NetBeans IDE 5.5 runs on operating systems that support the Java VM. • Microsoft Windows XP Professional SP2 or newer • Mac OS X 10.4.5 or newer • Red Hat Fedora Core 3 • Solaris™ 10 Operating System Update 1 (SPARC® and x86/x64 Platform Edition) NetBeans Enterprise Pack is also known to run on the following platforms: • Microsoft Windows 2000 Professional SP4 • Solaris™ 8 OS (SPARC and x86/x64 Platform Edition) and Solaris 9 OS (SPARC and x86/x64 Platform Edition) • Various other Linux distributions Minimum Hardware Configuration Note: The NetBeans IDE's minimum screen resolution is 1024x768 pixels. Operating System Processor Memory Disk Space Microsoft Windows 500 MHz Intel Pentium III 512 MB 850 MB of free disk space Linux 500 MHz Intel Pentium III workstation or equivalent 512 MB 450 MB of free disk space Solaris OS (SPARC) UltraSPARC II 450 MHz 512 MB 450 MB of free disk space Solaris OS (x86/x64 Platform Edition) AMD Opteron 100 Series 1.8 GHz 512 MB 450 MB of free disk space Macintosh OS X operating system PowerPC G4 512 MB 450 MB of free disk space Recommended Hardware Configuration Operating System Processor Memory Disk Speed Microsoft Windows 1.4 GHz Intel Pentium III workstation or equivalent 1 GB 1 GB of free disk space Linux 1.4 GHz Intel Pentium III workstation or equivalent 1 GB 850 MB of free disk space Solaris OS (SPARC) UltraSPARC IIIi 1 GHz 1 GB 850 MB of free disk space Solaris OS (x86/x64 Platform Edition) AMD Opteron 100 Series 1.8 GHz 1 GB 850 MB of free disk space Macintosh OS X operating system PowerPC G5 1 GB 850 MB of free disk space Required Software NetBeans Enterprise Pack 5.5 Early Access runs on the Java 2 Platform Standard Edition Development Kit 5.0 Update 1 or higher (JDK 5.0, version 1.5.0_01 or higher), which consists of the Java Runtime Environment plus developer tools for compiling, debugging, and running applications written in the Java language. Sun Java System Application Server Platform Edition 9 has been tested with JDK 5.0 update 6. • For Solaris, Windows, and Linux, you can download the JDK for your platform from http://java.sun.com/j2se/1.5.0/download.html • For Mac OS X, Java 2 Platform Standard Edition (J2SE) 5.0 Release 4, is required. You can download the JDK from Apple's Developer Connection site. Start here: http://developer.apple.com/java (you must register to download the JDK). For more information, please visit: http://www.netbeans.org/community/releases/40/relnotes.html Introduction to Programming I 2
- 3. J.E.D.I Table of Contents 1 Introduction to Computer Programming..............................................................10 1.1 Objectives............................................................................................... 10 1.2 Introduction............................................................................................. 10 1.3 Basic Components of a Computer................................................................11 1.3.1 Hardware.......................................................................................... 11 1.3.1.1 The Central Processing Unit.......................................................... 11 1.3.1.2 Memory .................................................................................... 11 1.3.1.3 Input and Output Devices............................................................. 12 1.3.2 Software........................................................................................... 12 1.4 Overview of Computer Programming Languages........................................... 13 1.4.1 What is a Programming Language?....................................................... 13 1.4.2 Categories of Programming Languages..................................................13 1.5 The Program Development Life Cycle...........................................................14 1.5.1 Problem Definition............................................................................. 15 1.5.2 Problem Analysis............................................................................... 15 1.5.3 Algorithm design and representation.................................................... 16 1.5.3.1 Flowcharting Symbols and their meanings...................................... 17 1.5.4 Coding and Debugging....................................................................... 18 1.6 Number Systems and Conversions.............................................................. 19 1.6.1 Decimal............................................................................................ 19 1.6.2 Binary.............................................................................................. 19 1.6.3 Octal................................................................................................ 19 1.6.4 Hexadecimal..................................................................................... 19 1.6.5 Conversions...................................................................................... 20 1.6.5.1 Decimal to Binary / Binary to Decimal............................................ 20 1.6.5.2 Decimal to Octal (or Hexadecimal)/Octal (or Hexadecimal) to Decimal.... 21 1.6.5.3 Binary to Octal / Octal to Binary.................................................... 22 1.6.5.4 Binary to Hexadecimal / Hexadecimal to Binary............................... 23 1.7 Exercises................................................................................................. 24 1.7.1 Writing Algorithms............................................................................. 24 1.7.2 Number Conversions.......................................................................... 24 2 Introduction to Java........................................................................................ 25 2.1 Objectives............................................................................................... 25 2.2 Java Background...................................................................................... 25 2.2.1 A little Bit of History .......................................................................... 25 2.2.2 What is Java Technology?................................................................... 25 2.2.2.1 A programming language............................................................. 25 2.2.2.2 A development environment......................................................... 25 2.2.2.3 An application environment.......................................................... 25 2.2.2.4 A deployment environment........................................................... 26 2.2.3 Some Features of Java........................................................................ 26 2.2.3.1 The Java Virtual Machine.............................................................. 26 2.2.3.2 Garbage Collection...................................................................... 26 2.2.3.3 Code Security............................................................................. 27 2.2.4 Phases of a Java Program....................................................................28 3 Getting to know your Programming Environment................................................. 29 3.1 Objectives............................................................................................... 29 3.2 Introduction............................................................................................. 29 3.3 My First Java Program............................................................................... 29 3.4 Using a Text Editor and Console................................................................. 30 Introduction to Programming I 3
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J.E.D.I 3.4.1 Errors .............................................................................................. 40 3.4.1.1 Syntax Errors............................................................................. 40 3.4.1.2 Run-time Errors.......................................................................... 41 3.5 Using NetBeans........................................................................................ 42 3.6 Exercises................................................................................................. 55 3.6.1 Hello World!...................................................................................... 55 3.6.2 The Tree........................................................................................... 55 4 Programming Fundamentals............................................................................. 56 4.1 Objectives............................................................................................... 56 4.2 Dissecting my first Java program................................................................ 56 4.3 Java Comments........................................................................................ 58 4.3.1 C++-Style Comments......................................................................... 58 4.3.2 C-Style Comments............................................................................. 58 4.3.3 Special Javadoc Comments................................................................. 58 4.4 Java Statements and blocks....................................................................... 59 4.5 Java Identifiers........................................................................................ 60 4.6 Java Keywords......................................................................................... 61 4.7 Java Literals............................................................................................ 62 4.7.1 Integer Literals ................................................................................. 62 4.7.2 Floating-Point Literals ........................................................................ 62 4.7.3 Boolean Literals ................................................................................ 62 4.7.4 Character Literals .............................................................................. 63 4.7.5 String Literals ................................................................................... 63 4.8 Primitive data types.................................................................................. 64 4.8.1 Logical - boolean............................................................................... 64 4.8.2 Textual – char................................................................................... 64 4.8.3 Integral – byte, short, int & long.......................................................... 65 4.8.4 Floating Point – float and double.......................................................... 66 4.9 Variables................................................................................................. 67 4.9.1 Declaring and Initializing Variables....................................................... 67 4.9.2 Outputting Variable Data.................................................................... 68 4.9.3 System.out.println() vs. System.out.print() ......................................... 68 4.9.4 Reference Variables vs. Primitive Variables............................................ 69 4.10 Operators.............................................................................................. 70 4.10.1 Arithmetic operators......................................................................... 70 4.10.2 Increment and Decrement operators................................................... 73 4.10.3 Relational operators......................................................................... 75 4.10.4 Logical operators.............................................................................. 78 4.10.4.1 && (logical AND) and & (boolean logical AND)............................... 79 4.10.4.2 || (logical OR) and | (boolean logical inclusive OR).........................81 4.10.4.3 ^ (boolean logical exclusive OR).................................................. 83 4.10.4.4 ! (logical NOT).......................................................................... 84 4.10.5 Conditional Operator (?:).................................................................. 85 4.10.6 Operator Precedence........................................................................ 87 4.11 Exercises............................................................................................... 88 4.11.1 Declaring and printing variables......................................................... 88 4.11.2 Getting the average of three numbers................................................. 88 4.11.3 Output greatest value....................................................................... 88 4.11.4 Operator precedence........................................................................ 88 5 Getting Input from the Keyboard....................................................................... 89 5.1 Objectives............................................................................................... 89 5.2 Using BufferedReader to get input.............................................................. 89 5.3 Using JOptionPane to get input................................................................... 93 Introduction to Programming I 4
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J.E.D.I 5.4 Exercises................................................................................................. 95 5.4.1 Last 3 words (BufferedReader version).................................................. 95 5.4.2 Last 3 words (JOptionPane version)...................................................... 95 6 Control Structures........................................................................................... 96 6.1 Objectives............................................................................................... 96 6.2 Decision Control Structures........................................................................ 96 6.2.1 if statement...................................................................................... 96 6.2.2 if-else statement............................................................................... 98 6.2.3 if-else-if statement........................................................................... 100 6.2.4 Common Errors when using the if-else statements:...............................101 6.2.5 Example for if-else-else if.................................................................. 102 6.2.6 switch statement............................................................................. 103 6.2.7 Example for switch........................................................................... 105 6.3 Repetition Control Structures....................................................................106 6.3.1 while loop....................................................................................... 106 6.3.2 do-while loop................................................................................... 108 6.3.3 for loop........................................................................................... 109 6.4 Branching Statements............................................................................. 110 6.4.1 break statement.............................................................................. 110 6.4.1.1 Unlabeled break statement......................................................... 110 6.4.1.2 Labeled break statement............................................................ 111 6.4.2 continue statement.......................................................................... 112 6.4.2.1 Unlabeled continue statement..................................................... 112 6.4.2.2 Labeled continue statement........................................................ 112 6.4.3 return statement.............................................................................. 113 6.5 Exercises............................................................................................... 114 6.5.1 Grades........................................................................................... 114 6.5.2 Number in words.............................................................................. 114 6.5.3 Hundred Times................................................................................ 114 6.5.4 Powers........................................................................................... 114 7 Java Arrays.................................................................................................. 115 7.1 Objectives............................................................................................. 115 7.2 Introduction to arrays.............................................................................. 115 7.3 Declaring Arrays..................................................................................... 116 7.4 Accessing an array element...................................................................... 118 7.5 Array length........................................................................................... 119 7.6 Multidimensional Arrays........................................................................... 120 7.7 Exercises............................................................................................... 121 7.7.1 Days of the Week............................................................................. 121 7.7.2 Greatest number.............................................................................. 121 7.7.3 Addressbook Entries......................................................................... 121 8 Command-line Arguments.............................................................................. 122 8.1 Objectives............................................................................................. 122 8.2 Command-line arguments........................................................................ 122 8.3 Command-line arguments in NetBeans...................................................... 124 8.4 Exercises............................................................................................... 128 8.4.1 Print arguments............................................................................... 128 8.4.2 Arithmetic Operations....................................................................... 128 9 Working with the Java Class Library................................................................. 129 9.1 Objectives............................................................................................. 129 9.2 Introduction to Object-Oriented Programming............................................. 129 9.3 Classes and Objects................................................................................ 130 9.3.1 Difference Between Classes and Objects.............................................. 130 Introduction to Programming I 5
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J.E.D.I 9.3.2 Encapsulation.................................................................................. 131 9.3.3 Class Variables and Methods.............................................................. 131 9.3.4 Class Instantiation........................................................................... 132 9.4 Methods................................................................................................ 133 9.4.1 What are Methods and Why Use Methods?........................................... 133 9.4.2 Calling Instance Methods and Passing Variables....................................134 9.4.3 Passing Variables in Methods............................................................. 135 9.4.3.1 Pass-by-value........................................................................... 135 9.4.3.2 Pass-by-reference...................................................................... 136 9.4.4 Calling Static Methods...................................................................... 137 9.4.5 Scope of a variable........................................................................... 138 9.5 Casting, Converting and Comparing Objects............................................... 141 9.5.1 Casting Primitive Types..................................................................... 141 9.5.2 Casting Objects............................................................................... 143 9.5.3 Converting Primitive Types to Objects and Vice Versa............................145 9.5.4 Comparing Objects........................................................................... 146 9.5.5 Determining the Class of an Object..................................................... 148 9.6 Exercises............................................................................................... 149 9.6.1 Defining terms................................................................................. 149 9.6.2 Java Scavenger Hunt........................................................................ 149 10 Creating your own Classes............................................................................ 150 10.1 Objectives............................................................................................ 150 10.2 Defining your own classes...................................................................... 151 10.3 Declaring Attributes.............................................................................. 152 10.3.1 Instance Variables.......................................................................... 152 10.3.2 Class Variables or Static Variables.................................................... 153 10.4 Declaring Methods................................................................................ 153 10.4.1 Accessor methods........................................................................... 154 10.4.2 Mutator Methods............................................................................ 155 10.4.3 Multiple Return statements.............................................................. 156 10.4.4 Static methods............................................................................... 156 10.4.5 Sample Source Code for StudentRecord class..................................... 157 10.5 The this reference................................................................................. 159 10.6 Overloading Methods............................................................................. 160 10.7 Declaring Constructors........................................................................... 162 10.7.1 Default Constructor........................................................................ 162 10.7.2 Overloading Constructors................................................................ 162 10.7.3 Using Constructors......................................................................... 163 10.7.4 The this() Constructor Call............................................................... 164 10.8 Packages............................................................................................. 165 10.8.1 Importing Packages........................................................................ 165 10.8.2 Creating your own packages............................................................ 165 10.8.3 Setting the CLASSPATH.................................................................. 166 10.9 Access Modifiers................................................................................... 168 10.9.1 default access (also called package accessibility)................................ 168 10.9.2 public access................................................................................. 168 10.9.3 protected access............................................................................ 169 10.9.4 private access................................................................................ 169 10.10 Exercises........................................................................................... 170 10.10.1 Address Book Entry....................................................................... 170 10.10.2 AddressBook................................................................................ 170 11 Inheritance, Polymorphism and Interfaces....................................................... 171 11.1 Objectives............................................................................................ 171 Introduction to Programming I 6
- 7. J.E.D.I 11.2 Inheritance.......................................................................................... 171 11.2.1 Defining Superclasses and Subclasses............................................... 172 11.2.2 The super keyword......................................................................... 174 11.2.3 Overriding Methods........................................................................ 175 11.2.4 Final Methods and Final Classes........................................................ 176 11.3 Polymorphism...................................................................................... 177 11.4 Abstract Classes................................................................................... 179 11.5 Interfaces............................................................................................ 181 11.5.1 Why do we use Interfaces?.............................................................. 181 11.5.2 Interface vs. Abstract Class..............................................................181 11.5.3 Interface vs. Class.......................................................................... 182 11.5.4 Creating Interfaces......................................................................... 182 11.5.5 Relationship of an Interface to a Class............................................... 184 11.5.6 Inheritance among Interfaces........................................................... 184 11.6 Exercises............................................................................................. 185 11.6.1 Extending StudentRecord................................................................ 185 11.6.2 The Shape abstract class................................................................. 185 12 Basic Exception Handling.............................................................................. 186 12.1 Objectives............................................................................................ 186 12.2 What are Exceptions?............................................................................ 186 12.3 Handling Exceptions.............................................................................. 186 12.4 Exercises............................................................................................. 189 12.4.1 Catching Exceptions1...................................................................... 189 12.4.2 Catching Exceptions 2..................................................................... 189 Appendix A : Java and NetBeans Installation.........................................................190 Installing Java in Ubuntu Dapper..................................................................... 191 Installing Java in Windows............................................................................. 196 Installing NetBeans in Ubuntu Dapper.............................................................. 200 Installing NetBeans in Windows....................................................................... 208 Appendix B: Getting to know your Programming Environment (Windows XP version).. 215 My First Java Program................................................................................... 215 Using a Text Editor and Console...................................................................... 216 Setting the Path........................................................................................ 229 Using NetBeans............................................................................................ 230 Appendix D : Machine Problems.......................................................................... 240 Machine Problem 1: Phone Book...................................................................... 240 Machine Problem 2: Minesweeper.................................................................... 241 Machine Problem 3: Number Conversion........................................................... 242 Introduction to Programming I 7
- 8. J.E.D.I Revision History For Version 1.3 June 2006 Section Details Appendix A and B, Chapter 3:Getting to know your programming environment Switch to Netbeans 5.5 Beta Version Appendix A, Chapter 3:Getting to know your programming environment Switch from Redhat Linux to Ubuntu Dapper Appendix F: Additional Exercises Added (Teacher's manual)-c/o JEDI member school teachers For Version 1.2 January 2006 Section Details Version Number Change from 1.1 to 1.2 Chapter 3: Gettting to know your programming environment Appendix A Change Netbeans/netbeans to NetBeans Chapter 4: Programming Fundamentals List of Java keywords Chapter 10: Creating your own classes Coding guidelines: filenames should have the same name as the public class name Master Documents Added to list of references For Version 1.1 August 2005 Section Details Version Number Change from 1.0 to 1.1 Revision History Added Appendix E: Hands-on Lab Exercises Added (c/o Sang) Chapter 10: Creating Your own classes Added subsection on How to set classpath at packages section Chapter 11: Inheritance, Interfaces and Polymorphism Polymorphism section • Added example that uses another class whose method can receive a reference variable Interface • Added sections • Why do we use Interfaces? Introduction to Programming I 8
- 9. J.E.D.I Section Details • Interface vs. Abstract Class • Interface vs. Class • Relationship of an Interface to a Class • Inheritance among Interfaces Introduction to Programming I 9
- 10. J.E.D.I 1 Introduction to Computer Programming 1.1 Objectives In this section, we will be discussing the basic components of a computer, both hardware and software. We will also be giving a brief overview of programming languages and the program development life cycle. Finally, different number systems and conversions from one type to another will be discussed. At the end of the lesson, the student should be able to: • Identify the different components of a computer • Know about programming languages and their categories • Understand the program development life cycle and apply it in problem solving • Learn the different number systems and their conversions 1.2 Introduction A computer is a machine that performs a variety of tasks according to specific instructions. It is a data processing machine which accepts data via an input device and its processor manipulates the data according to a program. The computer has two major components. The first one is the Hardware which is the tangible part of the computer. It is composed of electronic and mechanical parts. The second major component is the software which is the intangible part of a computer. It consists of data and the computer programs. Introduction to Programming I 10
- 11. J.E.D.I 1.3 Basic Components of a Computer 1.3.1 Hardware 1.3.1.1 The Central Processing Unit The processor is the “brain” of the computer. It contains millions of extremely tiny electrical parts. It does the fundamental computing within the system. Examples of processors are Pentium, Athlon and SPARC. 1.3.1.2 Memory The memory is where data and instructions needed by the CPU to do its appointed tasks can be found. It is divided into several storage locations which have corresponding addresses. The CPU accesses the memory with the use of these addresses. 1. Main Memory The main memory is very closely connected to the processor. It is used to hold programs and data, that the processor is actively working with. It is not used for long-term storage. It is sometimes called the RAM (Random Access Memory). The computer's main memory is considered as volatile storage. This means that once the computer is turned off, all information residing in the main memory is erased. 2. The Secondary Memory The secondary memory is connected to main memory. It is used to hold programs and data for long term use. Examples of secondary memory are hard disks and cd-rom. Secondary memory is considered as non-volatile storage. This means that information residing in secondary memory is not erased after the computer is turned off. Main Memory Secondary Memory Property Fast Slow Speed Expensive Cheap Price Low High Capacity Yes No Volatile Table 1: Comparison between main memory and secondary memory Introduction to Programming I 11
- 12. J.E.D.I 1.3.1.3 Input and Output Devices Input and output devices allows a computer system to interact with the outside world by moving data into and out of the system. Examples of input devices are keyboards, mice and microphones. Examples of output devices are monitors, printers and speakers. 1.3.2 Software A software is the program that a computer uses in order to function. It is kept on some hardware device like a hard disk, but it itself is intangible. The data that the computer uses can be anything that a program needs. Programs acts like instructions for the processor. Some Types of Computer Programs: 1. Systems Programs • Programs that are needed to keep all the hardware and software systems running together smoothly • Examples: • Operating Systems like Linux, Windows, Unix, Solaris, MacOS 2. Application Programs • Programs that people use to get their work done • Examples: • Word Processor • Game programs • Spreadsheets 3. Compilers • The computer understands only one language: machine language. Machine language is in the form of ones and zeros. Since it is highly impractical for people to create programs out of zeros and ones, there must be a way of translating or converting a language which we understand into machine language, for this purpose, there exists compilers. Introduction to Programming I 12
- 13. J.E.D.I 1.4 Overview of Computer Programming Languages 1.4.1 What is a Programming Language? A programming language is a standardized communication technique for expressing instructions to a computer. Like human languages, each language has its own syntax and grammar. Programming languages enable a programmer to precisely specify what data a computer will act upon, how these data will be stored/transmitted, and precisely what actions to take under various circumstances. There are different types of programming languages that can be used to create programs, but regardless of what language you use, these instructions are translated into machine language that can be understood by computers. 1.4.2 Categories of Programming Languages 1. High-level Programming Languages • A high-level programming language is a programming language that is more user- friendly, to some extent platform-independent, and abstract from low-level computer processor operations such as memory accesses. A programming statement may be translated into one or several machine instructions by a compiler. • Examples are Java, C, C++, Basic, Fortran 2. Low-level Assembly Language • Assembly languages are similar to machine languages, but they are much easier to program in because they allow a programmer to substitute names for numbers. Assembly languages are available for each CPU family, and each assembly instruction is translated into one machine instruction by an assembler program. Note: The terms "high-level" and "low-level" are inherently relative. Originally, assembly language was considered low-level and COBOL, C, etc. were considered high-level. Many programmers today might refer to these latter languages as low-level. Introduction to Programming I 13
- 14. J.E.D.I 1.5 The Program Development Life Cycle Programmers do not sit down and start writing code right away when trying to make a computer program. Instead, they follow an organized plan or methodology, that breaks the process into a series of tasks. Here are the basic steps in trying to solve a problem on the computer: 1. Problem Definition 2. Problem Analysis 3. Algorithm design and representation (Pseudocode or flowchart) 4. Coding and debugging In order to understand the basic steps in solving a problem on a computer, let us define a single problem that we will solve step-by-step as we discuss the problem solving methodologies in detail. The problem we will solve will be defined in the next section. Introduction to Programming I 14
- 15. J.E.D.I 1.5.1 Problem Definition A programmer is usually given a task in the form of a problem. Before a program can be designed to solve a particular problem, the problem must be well and clearly defined first in terms of its input and output requirements. A clearly defined problem is already half the solution. Computer programming requires us to define the problem first before we even try to create a solution. Let us now define our example problem: “Create a program that will determine the number of times a name occurs in a list.” 1.5.2 Problem Analysis After the problem has been adequately defined, the simplest and yet the most efficient and effective approach to solve the problem must be formulated. Usually, this step involves breaking up the problem into smaller and simpler sub- problems. Example Problem: Determine the number of times a name occurs in a list Input to the program: list of names, name to look for Output of the program: the number of times the name occurs in a list Introduction to Programming I 15
- 16. J.E.D.I 1.5.3 Algorithm design and representation Once our problem is clearly defined, we can now set to finding a solution. In computer programming, it is normally required to express our solution in a step-by-step manner. An Algorithm is a clear and unambiguous specification of the steps needed to solve a problem. It may be expressed in either Human language (English, Tagalog), through a graphical representation like a flowchart or through a pseudocode, which is a cross between human language and a programming language. Now given the problem defined in the previous sections, how do we express our general solution in such a way that it is simple yet understandable? Expressing our solution through Human language: 1. Get the list of names 2. Get the name to look for, let's call this the keyname 3. Compare the keyname to each of the names in the list 4. If the keyname is the same with a name in the list, add 1 to the count 5. If all the names have been compared, output the result Expressing our solution through a flowchart: Introduction to Programming I 16 YES Figure 1.1: Example of a flow chart
- 17. J.E.D.I Expressing our solution through pseudocode: Let nameList = List of Names Let keyName = the name to be sought Let Count = 0 For each name in NameList do the following if name == keyName Count = Count + 1 Display Count Figure 1.2: Example of a pseudocode 1.5.3.1 Flowcharting Symbols and their meanings A flowchart is a design tool used to graphically represent the logic in a solution. Flowcharts typically do not display programming language commands. Rather, they state the concept in English or mathematical notation. Here are some guidelines for commonly used symbols in creating flowcharts. You can use any symbols in creating your flowcharts, as long as you are consistent in using them. Symbol Name Meaning Process Symbol Represents the process of executing a defined operation or groups of operations that results in a change in value, form, or location of information. Also functions as the default symbol when no other symbol is available. Input/Output (I/O) Symbol Represents an I/O function, which makes data available for processing (input) or displaying (output)of processed information. Flowline Symbol Represents the sequence of available information and executable operations.The lines connect other symbols, and the arrowheads are mandatory only for right-to-left and bottom-to- top flow. Annotation Symbol Represents the addition of descriptive information, comments, or explanatory notes as clarification. The vertical line and the broken line may be placed on the left, as shown, or on the right. Decision Symbol Represents a decision that determines which of a number of alternative paths is to be followed. Terminal Symbol Represents the beginning, the end, or a point of interruption or delay in a program. Introduction to Programming I 17
- 18. J.E.D.I Symbol Name Meaning Connector Symbol Represents any entry from, or exit to, another part of the flowchart. Also serves as an off-page connector. Predefined Process Symbol Represents a named process consisting of one or more operations or program steps that are specified elsewhere. Table 2: Flowchart Symbols 1.5.4 Coding and Debugging After constructing the algorithm, it is now possible to create the source code. Using the algorithm as basis, the source code can now be written using the chosen programming language. Most of the time, after the programmer has written the program, the program isn't 100% working right away. The programmer has to add some fixes to the program in case of errors (also called bugs) that occurs in the program. This process of is called debugging. There are two types of errors that a programmer will encounter along the way. The first one is compile-time error, and the other is runtime error. Compile-Time Errors occur if there is a syntax error in the code. The compiler will detect the error and the program won't even compile. At this point, the programmer is unable to form an executable that a user can run until the error is fixed. Forgetting a semi-colon at the end of a statement or misspelling a certain command, for example, is a compile-time error. It's something the compiler can detect as an error. Compilers aren't perfect and so can't catch all errors at compile time. This is especially true for logic errors such as infinite loops. This type of error is called runtime error. For example, the actual syntax of the code looks okay. But when you follow the code's logic, the same piece of code keeps executing over and over again infinitely so that it loops. In such a case, compilers aren't really smart enough to catch all of these types of errors at compile-time, and therefore, the program compiles fine into an executable file. However, and unfortunately, when the end-user runs the program, the program (or even their whole computer) freezes up due to an infinite loop. Other types of run-time errors are when an incorrect value is computed, the wrong thing happens, etc. Introduction to Programming I 18
- 19. J.E.D.I 1.6 Number Systems and Conversions Numbers can be represented in a variety of ways. The representation depends on what is called the BASE. The following are the four most common representations. 1.6.1 Decimal We normally represent numbers in their decimal form. Numbers in decimal form are in base 10. This means that the only digits that appear are 0-9. Here are examples of numbers written in decimal form: 12610 (normally written as just 126) 1110 (normally written as just 11) 1.6.2 Binary Numbers in binary form are in base 2. This means that the only legal digits are 0 and 1. We need to write the subscript 2 to indicate that the number is a binary number. Here are examples of numbers written in binary form: 11111102 10112 1.6.3 Octal Numbers in octal form are in base 8. This means that the only legal digits are 0-7. We need to write the subscript 8 to indicate that the number is an octal number. Here are examples of numbers written in octal form: 1768 138 1.6.4 Hexadecimal Numbers in hexadecimal form are in base 16. This means that the only legal digits are 0- 9 and the letters A-F (or a-f, lowercase or uppercase does not matter). We need to write the subscript 16 to indicate that the number is a hexadecimal number. Here are examples of numbers written in hexadecimal form: 7E16 B16 Hexadecimal 0 1 2 3 4 5 6 7 8 9 A B C D E F Decimal Equivalent 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1 5 Table 3: Hexadecimal Numbers and their Equivalence to decimal numbers Decimal Binary Octal Hexadecimal 12610 11111102 1768 7E16 1110 10112 138 B16 Table 4: Summary of Examples Introduction to Programming I 19
- 20. J.E.D.I 1.6.5 Conversions 1.6.5.1 Decimal to Binary / Binary to Decimal To convert a decimal number to binary, continuously divide the number by 2 and get the remainder (which is either 0 or 1), and get that number as a digit of the binary form of the number. Get the quotient and divide that number again by 2 and repeat the whole process until the quotient reaches 0 or 1. We then get all the remainders starting from the last remainder, and the result is the binary form of the number. NOTE: For the last digit which is already less than the divisor (which is 2) just copy the value to the remainder portion. For Example: 12610 = ? 2 Quotient Remainder 126 / 2 = 63 0 63 / 2 = 31 1 31 / 2 = 15 1 15 / 2 = 7 1 7 / 2 = 3 1 3 / 2 = 1 1 1 / 2 = 1 Writeitthisway So, writing the remainders from the bottom up, we get the binary number 11111102 To convert a binary number to decimal, we multiply the binary digit to "2 raised to the position of the binary number". We then add all the products to get the resulting decimal number. For Example: 11111102 = ? 10 Position 6 5 4 3 2 1 0 Binary Digits 1 1 1 1 1 1 0 0 x 20 = 0 1 x 21 = 2 1 x 22 = 4 1 x 23 = 8 1 x 24 = 16 1 x 25 = 32 1 x 26 = 64 TOTAL: 126 Introduction to Programming I 20
- 21. J.E.D.I 1.6.5.2 Decimal to Octal (or Hexadecimal)/Octal (or Hexadecimal) to Decimal Converting decimal numbers to Octal or hexadecimal is basically the same as converting decimal to binary. However, instead of having 2 as the divisor, you replace it with 8(for octal) or 16 (for hexadecimal). For Example (Octal): 12610 = ? 8 Quotient Remainder 126 / 8 = 15 6 15 / 8 = 1 7 1 / 8 = 1 Writeitthisway So, writing the remainders from the bottom up, we get the octal number 1768 For Example (Hexadecimal): 12610 = ? 16 Quotient Remainder 126 / 16 = 7 14 (equal to hex digit E) 7 / 16 = 7 WriteitthiswaySo, writing the remainders from the bottom up, we get the hexadecimal number 7E16 * * * Converting octal or hexadecimal numbers is also the same as converting binary numbers to decimal. To do that, we will just replace the base number 2 with 8 for Octal and 16 for hexadecimal. For Example (Octal): 1768 = ? 10 Position 2 1 0 Octal Digits 1 7 6 6 x 80 = 6 7 x 81 = 56 1 x 82 = 64 TOTAL: 126 Introduction to Programming I 21
- 22. J.E.D.I For Example (Hexadecimal): 7E16 = ? 10 Position 1 0 Hex Digits 7 E 14 x 160 = 14 7 x 161 = 112 TOTAL: 126 1.6.5.3 Binary to Octal / Octal to Binary To convert from binary numbers to octal, we partition the binary number into groups of 3 digits (from right to left), and pad it with zeros if the number of digits is not divisible by 3. We then convert each partition into its corresponding octal digit. The following is a table showing the binary representation of each octal digit. Octal Digit Binary Representation 0 000 1 001 2 010 3 011 4 100 5 101 6 110 7 111 Table 5: Octal Digits and their corresponding binary represenation For Example: 11111102 = ? 8 0 0 1 1 1 1 1 1 0 1 7 6 Equivalent octal number Converting octal numbers to binary is just the opposite of what is given above. Simply convert each octal digit into its binary representation (given the table) and concatenate them. The result is the binary representation. Introduction to Programming I 22
- 23. J.E.D.I 1.6.5.4 Binary to Hexadecimal / Hexadecimal to Binary To convert from binary numbers to hexadecimal, we partition the binary number into groups of 4 digits (from right to left), and pad it with zeros if the number of digits is not divisible by 4. We then convert each partition into its corresponding hexadecimal digit. The following is a table showing the binary representation of each hexadecimal digit. Hexadecimal Digit Binary Representation 0 0000 1 0001 2 0010 3 0011 4 0100 5 0101 6 0110 7 0111 8 1000 9 1001 A 1010 B 1011 C 1100 D 1101 E 1110 F 1111 Table 6: Hexadecimal Digits and their corresponding binary represenation For Example: 11111102 = ? 16 0 1 1 1 1 1 1 0 7 E Equivalent Hexadecimal number Converting hexadecimal numbers to binary is just the opposite of what is given above. Simply convert each hexadecimal digit into its binary representation (given the table) and concatenate them. The result is the binary representation. Introduction to Programming I 23
- 24. J.E.D.I 1.7 Exercises 1.7.1 Writing Algorithms Given the following set of tasks, create an algorithm to accomplish the following tasks. You may write your algorithms using pseudocodes or you can use flowcharts. 1. Baking Bread 2. Logging into your laboratory's computer 3. Getting the average of three numbers 1.7.2 Number Conversions Convert the following numbers: 1. 198010 to binary, hexadecimal and octal 2. 10010011012 to decimal, hexadecimal and octal 3. 768 to binary, hexadecimal and decimal 4. 43F16 to binary, decimal and octal Introduction to Programming I 24
- 25. J.E.D.I 2 Introduction to Java 2.1 Objectives In this section, we will be discussing a little bit of Java history and what is Java Technology. We will also discuss the phases that a Java program undergoes. At the end of the lesson, the student should be able to: • Describe the features of Java technology such as the Java virtual machine, garbage collection and code security • Describe the different phases of a Java program 2.2 Java Background 2.2.1 A little Bit of History Java was created in 1991 by James Gosling et al. of Sun Microsystems. Initially called Oak, in honor of the tree outside Gosling's window, its name was changed to Java because there was already a language called Oak. The original motivation for Java was the need for platform independent language that could be embedded in various consumer electronic products like toasters and refrigerators. One of the first projects developed using Java was a personal hand-held remote control named Star 7. At about the same time, the World Wide Web and the Internet were gaining popularity. Gosling et. al. realized that Java could be used for Internet programming. 2.2.2 What is Java Technology? 2.2.2.1 A programming language As a programming language, Java can create all kinds of applications that you could create using any conventional programming language. 2.2.2.2 A development environment As a development environment, Java technology provides you with a large suite of tools: a compiler, an interpreter, a documentation generator, a class file packaging tool, and so on. 2.2.2.3 An application environment Java technology applications are typically general-purpose programs that run on any machine where the Java runtime environment (JRE) is installed. Introduction to Programming I 25
- 26. J.E.D.I 2.2.2.4 A deployment environment There are two main deployment environments: First, the JRE supplied by the Java 2 Software Development Kit (SDK) contains the complete set of class files for all the Java technology packages, which includes basic language classes, GUI component classes, and so on. The other main deployment environment is on your web browser. Most commercial browsers supply a Java technology interpreter and runtime environment. 2.2.3 Some Features of Java 2.2.3.1 The Java Virtual Machine The Java Virtual Machine is an imaginary machine that is implemented by emulating software on a real machine. The JVM provides the hardware platform specifications to which you compile all Java technology code. This specification enables the Java software to be platform-independent because the compilation is done for a generic machine known as the JVM. A bytecode is a special machine language that can be understood by the Java Virtual Machine (JVM). The bytecode is independent of any particular computer hardware, so any computer with a Java interpreter can execute the compiled Java program, no matter what type of computer the program was compiled on. 2.2.3.2 Garbage Collection Many programming languages allows a programmer to allocate memory during runtime. However, after using that allocated memory, there should be a way to deallocate that memory block in order for other programs to use it again. In C, C++ and other languages the programmer is responsible for this. This can be difficult at times since there can be instances wherein the programmers forget to deallocate memory and therefor result to what we call memory leaks. In Java, the programmer is freed from the burden of having to deallocate that memory themselves by having what we call the garbage collection thread. The garbage collection thread is responsible for freeing any memory that can be freed. This happens automatically during the lifetime of the Java program. Introduction to Programming I 26
- 27. J.E.D.I 2.2.3.3 Code Security Code security is attained in Java through the implementation of its Java Runtime Environment (JRE). The JRE runs code compiled for a JVM and performs class loading (through the class loader), code verification (through the bytecode verifier) and finally code execution. The Class Loader is responsible for loading all classes needed for the Java program. It adds security by separating the namespaces for the classes of the local file system from those that are imported from network sources. This limits any Trojan horse applications since local classes are always loaded first. After loading all the classes, the memory layout of the executable is then determined. This adds protection against unauthorized access to restricted areas of the code since the memory layout is determined during runtime. After loading the class and layouting of memory, the bytecode verifier then tests the format of the code fragments and checks the code fragments for illegal code that can violate access rights to objects. After all of these have been done, the code is then finally executed. Introduction to Programming I 27
- 28. J.E.D.I 2.2.4 Phases of a Java Program The following figure describes the process of compiling and executing a Java program. The first step in creating a Java program is by writing your programs in a text editor. Examples of text editors you can use are notepad, vi, emacs, etc. This file is stored in a disk file with the extension .java. After creating and saving your Java program, compile the program by using the Java Compiler. The output of this process is a file of Java bytecodes with the file extension .class. The .class file is then interpreted by the Java interpreter that converts the bytecodes into the machine language of the particular computer you are using. Task Tool to use Output Write the program Any text editor File with .java extension Compile the program Java Compiler File with .class extension (Java bytecodes) Run the program Java Interpreter Program Output Table 7: Summary of Phases of a Java Program Introduction to Programming I 28 Figure 2.1: Phases of a Java Program
- 29. J.E.D.I 3 Getting to know your Programming Environment 3.1 Objectives In this section, we will be discussing on how to write, compile and run Java programs. There are two ways of doing this, the first one is by using a console and a text editor. The second one is by using NetBeans which is an Integrated Development Environment or IDE. At the end of the lesson, the student should be able to: • Create a Java program using text editor and console in the Linux (Ubuntu Dapper) environment • Differentiate between syntax-errors and runtime errors • Create a Java program using NetBeans 3.2 Introduction An IDE is a programming environment integrated into a software application that provides a GUI builder, a text or code editor, a compiler and/or interpreter and a debugger. This tutorial uses Ubuntu Dapper as the operating system. Make sure that before you do this tutorial, you have installed Java and NetBeans in your system. For instructions on how to install Java and NetBeans, please refer to Appendix A. For the Windows XP version of this section, please refer to Appendix B. Before going into details, let us first take a look at the first Java program you will be writing. 3.3 My First Java Program public class Hello { /** * My first java program */ public static void main(String[] args) { //prints the string "Hello world" on screen System.out.println("Hello world!"); } } Before we try to explain what the program means, let's first try to write this program in a file and try to run it. Introduction to Programming I 29
- 30. J.E.D.I 3.4 Using a Text Editor and Console For this example, we will be using a text editor to edit the Java source code. You will also need to open the Terminal window to compile and execute your Java programs. Step 1: Start the Text Editor To start the Text Editor in Linux, click on Applications->Accessories->Text Editor. Step 2: Open Terminal To open Terminal in Linux, click on Applications-> Accessories-> Terminal. Introduction to Programming I 30 Figure 3.1: Text Editor Application in Linux Figure 3.2: Terminal in Linux
- 31. J.E.D.I Step 3: Write your the source code of your Java program in the Text Editor Introduction to Programming I 31 Figure 3.3: Writing the Source Code with the Text Editor
- 32. J.E.D.I Step 4: Save your Java Program We will save our program on a file named "Hello.java", and we will be saving it inside a folder named MYJAVAPROGRAMS. To open the Save dialog box, click on the File menu found on the menubar and then click on Save. After doing the procedure described above, a dialog box will appear as shown in Figure below. Introduction to Programming I 32 Figure 3.4: Save As Dialog
- 33. J.E.D.I Click on the browse button, and then click on the Create Folder button. Introduction to Programming I 33
- 34. J.E.D.I Name the new folder MYJAVAPROGRAMS. Now, click on the MYJAPROGRAMS folder in order to get inside that folder. You will see a similar figure as shown below after you clicked on MYJAVAPROGRAMS. The folder should be empty for now since it's a newly created folder and we haven't saved anything in it yet. Now, in the Selection textbox, type in the filename of your program, which is "Hello.java", and then click on the SAVE button. Introduction to Programming I 34
- 35. J.E.D.I Now that you've saved your file, notice how the title of the frame changes from "Untitled Document 1 (modified) – gedit" to "Hello.java (~/MYJAVAPROGRAMS) - gedit". Take note that if you want to make changes in your file, you can just edit it, and then save it again by clicking on File -> Save. Introduction to Programming I 35 Figure 3.5: New Window After Saving
- 36. J.E.D.I Step 5: Compiling your program Now, the next step is to compile your program. Go to the Terminal window we just opened a while ago. Typically, when you open the terminal window, it opens up and takes you directly to what is called your home folder. To see what is inside that home folder, type ls and then press ENTER. What you will see is a list of files and folders inside your home folder. Now, you can see here that there is a folder named "MYJAVAPROGRAMS" which we have created a while ago, and where we saved our Hello.java program. Now let's go inside that directory. To go inside a directory, you type in the command: cd [directory name]. The "cd" command stands for, change directory. In this case, since the name of our directory is MYJAVAPROGRAMS, you type in: cd MYJAVAPROGRAMS Introduction to Programming I 36 Figure 3.6: Changing the Directory
- 37. J.E.D.I Once inside the folder where your Java programs are, let us now start compiling your Java program. Take note that, you should make sure that the file is inside the folder where you are in. In order to do that, execute the "ls" command again to see if your file is inside that folder. Introduction to Programming I 37 Figure 3.7: List of Files Inside the New Directory
- 38. J.E.D.I To compile a Java program, we type in the command: javac [filename]. So in this case, type in: javac Hello.java. During compilation, javac adds a file to the disk called [filename].class, or in this case, Hello.class, which is the actual bytecode. Introduction to Programming I 38 Figure 3.8: Compiling Java File
- 39. J.E.D.I Step 6: Running the Program Now, assuming that there are no problems during compilation (we'll explore more of the problems encountered during compilation in the next section), we are now ready to run your program. To run your Java program, type in the command: java [filename without the extension], so in the case of our example, type in: java Hello You can see on the screen that you have just run your first Java program that prints the message, "Hello world!". Introduction to Programming I 39 Figure 3.9: Running Class File
- 40. J.E.D.I 3.4.1 Errors What we've shown so far is a Java program wherein we didn't encounter any problems in compiling and running. However, this is not always the case. As what we have discussed in the first part of this course, we usually encounter errors along the way. As discussed before, there are two types of errors. The first one is a compile-time error or also called as syntax error. The second one is the runtime error. 3.4.1.1 Syntax Errors Syntax errors are usually typing errors. You may have misspelled a command in Java or forgot to write a semi-colon at the end of a statement. Java attempts to isolate the error by displaying the line of code and pointing to the first incorrect character in that line. However, the problem may not be at the exact point. Other common mistakes are in capitalization, spelling, the use of incorrect special characters, and omission of correct punctuation. Let's take for example, our Hello.java program wherein we intentionally omit the semicolon at one statement and we try to type the incorrect spelling of a command. Introduction to Programming I 40 Figure 3.10: Source Code With Errors
- 41. J.E.D.I See the error messages generated after compiling the program. The first error message suggests that there is an error in line 6 of your program. It pointed to the next word after the statict, which should be spelled as static. The second error message suggests that there is a missing semicolon after your statement. As a rule of thumb, if you encounter a lot of error messages, try to correct the first mistake in a long list, and try to compile the program again. Doing so may reduce the total number of errors dramatically. 3.4.1.2 Run-time Errors Run-time errors are errors that will not display until you run or execute your program. Even programs that compile successfully may display wrong answers if the programmer has not thought through the logical processes and structures of the program. Introduction to Programming I 41 Figure 3.11: Compiling the Source Code with Errors
- 42. J.E.D.I 3.5 Using NetBeans Now that we've tried doing our programs the complicated way, let's now see how to do all the processes we've described in the previous sections by using just one application. In this part of the lesson, we will be using NetBeans, which is an Integrated Development Environment or IDE. An IDE is a programming environment integrated into a software application that provides a GUI builder, a text or code editor, a compiler and/or interpreter and a debugger. Step 1: Run NetBeans There are two ways to run NetBeans. One is through command-line using terminal, or by jst clicking on the shortcut button found on the desktop. To run NetBeans using command-line. Open terminal (see steps on how to run terminal in the previous discussion), and type: netbeans Introduction to Programming I 42 Figure 3.12: Running NetBeans with the Command-Line
- 43. J.E.D.I The second way to run NetBeans, is by clicking on the shortcut icon found on your Desktop. Introduction to Programming I 43 Figure 3.13: Running NetBeans using shortcut icon on desktop
- 44. J.E.D.I After you've open NetBeans IDE, you will see a graphical user interface (GUI) similar to what is shown below. Introduction to Programming I 44 Figure 3.14: Window After Openning NetBeans
- 45. J.E.D.I Step 2: Make a project Now, let's first make a project. Click on File-> New Project. After doing this, a New Project dialog will appear. Now click on Java Application and click on the NEXT button. Introduction to Programming I 45 Figure 3.15: Choosing Project Type
- 46. J.E.D.I Now, a New Application dialog will appear. Introduction to Programming I 46 Figure 3.16: Setting the Project Information
- 47. J.E.D.I Now try to change the Application Location, by clicking on the BROWSE button. A Project Location dialog will then appear. Double-click on your home folder. The contents of the root folder is then displayed. Now double-click on the MYJAVAPROGRAMS folder and click on the OPEN button. Introduction to Programming I 47 Figure 3.17: Setting the Project Location
- 48. J.E.D.I See now that the Project Location and Project Folder is changed to /home/florence/MYJAVAPROGRAMS. Finally, on the Create Main Class textfield, type in Hello as the main class' name, and then click on the FINISH button. Introduction to Programming I 48 Figure 3.18: Window after Setting the Project Location to MYJAVAPROGRAMS/Setting the Main Class of the Project to Hello
- 49. J.E.D.I Step 3: Type in your program Before typing in your program, let us first describe the main window after creating the project. As shown below, NetBeans automatically creates the basic code for your Java program. You can just add your own statements to the generated code. On the left side of the window, you can see a list of folders and files that NetBeans generated after creating the project. This can all be found in your MYJAVAPROGRAMS folder, where you set the Project location. Introduction to Programming I 49 Figure 3.19: View of the Created Project
- 50. J.E.D.I Now, try to modify the code generated by NetBeans. Ignore the other parts of the program for now, as we will explain the details of the code later. Insert the code: System.out.println("Hello world!"); after the statement, //TODO code application logic here. Introduction to Programming I 50 Figure 3.20: Inserting the Code
- 51. J.E.D.I Step 4: Compile your program Now, to compile your program, just click on Build -> Build Main Project. Or, you could also use the shortcut button to compile your code. Introduction to Programming I 51
- 52. J.E.D.I If there are no errors in your program, you will see a build successful message on the output window. Introduction to Programming I 52 Figure 3.21: View after a Successful Compilation
- 53. J.E.D.I Step 5: Run your program To run your program, click on Run-> Run Main Project. Or you could also use the shortcut button to run your program. Introduction to Programming I 53 Figure 3.22: Running with NetBeans
- 54. J.E.D.I The output of your program is displayed in the output window. Introduction to Programming I 54 Figure 3.23: View after a Successful Run
- 55. J.E.D.I 3.6 Exercises 3.6.1 Hello World! Using NetBeans, create a class named: [YourName]. The program should output on the screen: Welcome to Java Programming [YourName]!!! 3.6.2 The Tree Using NetBeans, create a class named: TheTree. The program should output the following lines on the screen: I think that I shall never see, a poem as lovely as a tree. A tree whose hungry mouth is pressed Against the Earth’s sweet flowing breast. Introduction to Programming I 55
- 56. J.E.D.I 4 Programming Fundamentals 4.1 Objectives In this section, we will be discussing the basic parts of a Java program. We will start by trying to explain the basic parts of the Hello.java program introduced in the previous section. We will also be discussing some coding guidelines or code conventions along the way to help in effectively writing readable programs. At the end of the lesson, the student should be able to: • Identify the basic parts of a Java program • Differentiate among Java literals, primitive data types, variable types ,identifiers and operators • Develop a simple valid Java program using the concepts learned in this chapter 4.2 Dissecting my first Java program Now, we'll try to the dissect your first Java program: public class Hello { /** * My first java program */ public static void main(String[] args) { //prints the string "Hello world" on screen System.out.println("Hello world!"); } } The first line of the code, public class Hello indicates the name of the class which is Hello. In Java, all code should be placed inside a class declaration. We do this by using the class keyword. In addition, the class uses an access specifier public, which indicates that our class in accessible to other classes from other packages (packages are a collection of classes). We will be covering packages and access specifiers later. The next line which contains a curly brace { indicates the start of a block. In this code, we placed the curly brace at the next line after the class declaration, however, we can also place this next to the first line of our code. So, we could actually write our code as: public class Hello { or public class Hello { Introduction to Programming I 56
- 57. J.E.D.I The next three lines indicates a Java comment. A comment is something used to document a part of a code. It is not part of the program itself, but used for documentation purposes. It is good programming practice to add comments to your code. /** * My first java program */ A comment is indicated by the delimiters “/*” and “*/”. Anything within these delimiters are ignored by the Java compiler, and are treated as comments. The next line, public static void main(String[] args) { or can also be written as, public static void main(String[] args) { indicates the name of one method in Hello which is the main method. The main method is the starting point of a Java program. All programs except Applets written in Java start with the main method. Make sure to follow the exact signature. The next line is also a Java comment, //prints the string "Hello world" on screen Now, we learned two ways of creating comments. The first one is by placing the comment inside /* and */, and the other one is by writing // at the start of the comment. The next line, System.out.println("Hello world!"); prints the text “Hello World!” on screen. The command System.out.println(), prints the text enclosed by quotation on the screen. The last two lines which contains the two curly braces is used to close the main method and class respectively. Coding Guidelines: 1. Your Java programs should always end with the .java extension. 2. Filenames should match the name of your public class. So for example, if the name of your public class is Hello, you should save it in a file called Hello.java. 3. You should write comments in your code explaining what a certain class does, or what a certain method do. Introduction to Programming I 57
- 58. J.E.D.I 4.3 Java Comments Comments are notes written to a code for documentation purposes. Those text are not part of the program and does not affect the flow of the program. Java supports three types of comments: C++-style single line comments, C-style multiline comments and special javadoc comments. 4.3.1 C++-Style Comments C++ Style comments starts with //. All the text after // are treated as comments. For example, // This is a C++ style or single line comments 4.3.2 C-Style Comments C-style comments or also called multiline comments starts with a /* and ends with a */. All text in between the two delimeters are treated as comments. Unlike C++ style comments, it can span multiple lines. For example, /* this is an exmaple of a C style or multiline comments */ 4.3.3 Special Javadoc Comments Special Javadoc comments are used for generating an HTML documentation for your Java programs. You can create javadoc comments by starting the line with /** and ending it with */. Like C-style comments, it can also span lines. It can also contain certain tags to add more information to your comments. For example, /** This is an example of special java doc comments used for n generating an html documentation. It uses tags like: @author Florence Balagtas @version 1.2 */ Introduction to Programming I 58
- 59. J.E.D.I 4.4 Java Statements and blocks A statement is one or more lines of code terminated by a semicolon. An example of a single statement is, System.out.println(“Hello world”); A block is one or more statements bounded by an opening and closing curly braces that groups the statements as one unit. Block statements can be nested indefinitely. Any amount of white space is allowed. An example of a block is, public static void main( String[] args ){ System.out.println("Hello"); System.out.println("world"); } Coding Guidelines: 1. In creating blocks, you can place the opening curly brace in line with the statement, like for example, public static void main( String[] args ){ or you can place the curly brace on the next line, like, public static void main( String[] args ) { 2. You should indent the next statements after the start of a block,for example, public static void main( String[] args ){ System.out.println("Hello"); System.out.println("world"); } Introduction to Programming I 59
- 60. J.E.D.I 4.5 Java Identifiers Identifiers are tokens that represent names of variables, methods, classes, etc. Examples of identifiers are: Hello, main, System, out. Java identifiers are case-sensitive. This means that the identifier: Hello is not the same as hello. Identifiers must begin with either a letter, an underscore “_”, or a dollar sign “$”. Letters may be lower or upper case. Subsequent characters may use numbers 0 to 9. Identifiers cannot use Java keywords like class, public, void, etc. We will discuss more about Java keywords later. Coding Guidelines: 1. For names of classes, capitalize the first letter of the class name. For names of methods and variables, the first letter of the word should start with a small letter.For example: ThisIsAnExampleOfClassName thisIsAnExampleOfMethodName 2. In case of multi-word identifiers, use capital letters to indicate the start of the word except the first word. For example, charArray, fileNumber, ClassName. 3. Avoid using underscores at the start of the identifier such as _read or _write. Introduction to Programming I 60
- 61. J.E.D.I 4.6 Java Keywords Keywords are predefined identifiers reserved by Java for a specific purpose. You cannot use keywords as names for your variables, classes, methods …etc. Here is a list of the Java Keywords. We will try to discuss all the meanings of these keywords and how they are used in our Java programs as we go along the way. Note: true, false, and null are not keywords but they are reserved words, so you cannot use them as names in your programs either Introduction to Programming I 61 Figure 4.1: Java Key Words
- 62. J.E.D.I 4.7 Java Literals Literals are tokens that do not change or are constant. The different types of literals in Java are: Integer Literals, Floating-Point Literals, Boolean Literals, Character Literals and String Literals. 4.7.1 Integer Literals Integer literals come in different formats: decimal (base 10), hexadecimal (base 16), and octal (base 8). In using integer literals in our program, we have to follow some special notations. For decimal numbers, we have no special notations. We just write a decimal number as it is. For hexadecimal numbers, it should be preceeded by “0x” or “0X”. For octals, they are preceeded by “0”. For example, consider the number 12. It's decimal representation is 12, while in hexadecimal, it is 0xC, and in octal, it is equivalent to 014. Integer literals default to the data type int. An int is a signed 32-bit value. In some cases, you may wish to force integer literal to the data type long by appending the “l” or “L” character. A long is a signed 64-bit value. We will cover more on data types later. 4.7.2 Floating-Point Literals Floating point literals represent decimals with fractional parts. An example is 3.1415. Floating point literals can be expressed in standard or scientific notations. For example, 583.45 is in standard notation, while 5.8345e2 is in scientific notation. Floating point literals default to the data type double which is a 64-bit value. To use a smaller precision (32-bit) float, just append the “f” or “F” character. 4.7.3 Boolean Literals Boolean literals have only two values, true or false. Introduction to Programming I 62
- 63. J.E.D.I 4.7.4 Character Literals Character Literals represent single Unicode characters. A Unicode character is a 16-bit character set that replaces the 8-bit ASCII character set. Unicode allows the inclusion of symbols and special characters from other languages. To use a character literal, enclose the character in single quote delimiters. For example, the letter a, is represented as ‘a’. To use special characters such as a newline character, a backslash is used followed by the character code. For example, ‘n’ for the newline character, ‘r’ for the carriage return, ‘b’ for backspace. 4.7.5 String Literals String literals represent multiple characters and are enclosed by double quotes. An example of a string literal is, “Hello World”. Introduction to Programming I 63
- 64. J.E.D.I 4.8 Primitive data types The Java programming language defines eight primitive data types. The following are, boolean (for logical), char (for textual), byte, short, int, long (integral), double and float (floating point). 4.8.1 Logical - boolean A boolean data type represents two states: true and false. An example is, boolean result = true; The example shown above, declares a variable named result as boolean type and assigns it a value of true. 4.8.2 Textual – char A character data type (char), represents a single Unicode character. It must have its literal enclosed in single quotes(’ ’). For example, ‘a’ //The letter a ‘t’ //A tab To represent special characters like ' (single quotes) or " (double quotes), use the escape character . For example, ''' //for single quotes '"' //for double quotes Although, String is not a primitive data type (it is a Class), we will just introduce String in this section. A String represents a data type that contains multiple characters. It is not a primitive data type, it is a class. It has it’s literal enclosed in double quotes(“”). For example, String message=“Hello world!” Introduction to Programming I 64
- 65. J.E.D.I 4.8.3 Integral – byte, short, int & long Integral data types in Java uses three forms – decimal, octal or hexadecimal. Examples are, 2 //The decimal value 2 077 //The leading 0 indicates an octal value 0xBACC //The leading 0x indicates a hexadecimal value Integral types has int as default data type. You can define its long value by appending the letter l or L. Integral data type have the following ranges: Integer Length Name or Type Range 8 bits byte -27 to 27 -1 16 bits short -215 to 215 -1 32 bits int -231 to 231 -1 64 bits long -263 to 263 -1 Table 8: Integral types and their ranges Coding Guidelines: In defining a long value, a lowercase L is not recommended because it is hard to distinguish from the digit 1. Introduction to Programming I 65
- 66. J.E.D.I 4.8.4 Floating Point – float and double Floating point types has double as default data type. Floating-point literal includes either a decimal point or one of the following, E or e //(add exponential value) F or f //(float) D or d //(double) Examples are, 3.14 //A simple floating-point value (a double) 6.02E23 //A large floating-point value 2.718F //A simple float size value 123.4E+306D //A large double value with redundant D In the example shown above, the 23 after the E in the second example is implicitly positive. That example is equivalent to 6.02E+23. Floating-point data types have the following ranges: Float Length Name or Type Range 32 bits float -231 to 231 -1 64 bits double -263 to 263 -1 Table 9: Floating point types and their ranges Introduction to Programming I 66
- 67. J.E.D.I 4.9 Variables A variable is an item of data used to store state of objects. A variable has a data type and a name. The data type indicates the type of value that the variable can hold. The variable name must follow rules for identifiers. 4.9.1 Declaring and Initializing Variables To declare a variable is as follows, <data type> <name> [=initial value]; Note: Values enclosed in <> are required values, while those values enclosed in [] are optional. Here is a sample program that declares and initializes some variables, public class VariableSamples { public static void main( String[] args ){ //declare a data type with variable name // result and boolean data type boolean result; //declare a data type with variable name // option and char data type char option; option = 'C'; //assign 'C' to option //declare a data type with variable name //grade, double data type and initialized //to 0.0 double grade = 0.0; } } Coding Guidelines: 1. It always good to initialize your variables as you declare them. 2. Use descriptive names for your variables. Like for example, if you want to have a variable that contains a grade for a student, name it as, grade and not just some random letters you choose. 3. Declare one variable per line of code. For example, the variable declarations, double exam=0; double quiz=10; double grade = 0; is preferred over the declaration, double exam=0, quiz=10, grade=0; Introduction to Programming I 67
- 68. J.E.D.I 4.9.2 Outputting Variable Data In order to output the value of a certain variable, we can use the following commands, System.out.println() System.out.print() Here's a sample program, public class OutputVariable { public static void main( String[] args ){ int value = 10; char x; x = ‘A’; System.out.println( value ); System.out.println( “The value of x=“ + x ); } } The program will output the following text on screen, 10 The value of x=A 4.9.3 System.out.println() vs. System.out.print() What is the difference between the commands System.out.println() and System.out.print()? The first one appends a newline at the end of the data to output, while the latter doesn't. Consider the statements, System.out.print("Hello "); System.out.print("world!"); These statements will output the following on the screen, Hello world! Now consider the following statements, System.out.println("Hello "); System.out.println("world!"); These statements will output the following on the screen, Hello world! Introduction to Programming I 68
- 69. J.E.D.I 4.9.4 Reference Variables vs. Primitive Variables We will now differentiate the two types of variables that Java programs have. These are reference variables and primitive variables. Primitive variables are variables with primitive data types. They store data in the actual memory location of where the variable is. Reference variables are variables that stores the address in the memory location. It points to another memory location of where the actual data is. When you declare a variable of a certain class, you are actually declaring a reference variable to the object with that certain class. For example, suppose we have two variables with data types int and String. int num = 10; String name = "Hello" Suppose, the illustration shown below is the actual memory of your computer, wherein you have the address of the memory cells, the variable name and the data they hold. Memory Address Variable Name Data 1001 num 10 : : 1563 name Address(2000) : : : : 2000 "Hello" As you can see, for the primitive variable num, the data is on the actual location of where the variable is. For the reference variable name, the variable just holds the address of where the actual data is. Introduction to Programming I 69
- 70. J.E.D.I 4.10 Operators In Java, there are different types of operators. There are arithmetic operators, relational operators, logical operators and conditional operators. These operators follow a certain kind of precedence so that the compiler will know which operator to evaluate first in case multiple operators are used in one statement. 4.10.1 Arithmetic operators Here are the basic arithmetic operators that can be used in creating your Java programs, Operator Use Description + op1 + op2 Adds op1 and op2 * op1 * op2 Multiplies op1 by op2 / op1 / op2 Divides op1 by op2 % op1 % op2 Computes the remainder of dividing op1 by op2 - op1 - op2 Subtracts op2 from op1 Table 10: Arithmetic operations and their functions Introduction to Programming I 70
- 71. J.E.D.I Here's a sample program in the usage of these operators: public class ArithmeticDemo { public static void main(String[] args) { //a few numbers int i = 37; int j = 42; double x = 27.475; double y = 7.22; System.out.println("Variable values..."); System.out.println(" i = " + i); System.out.println(" j = " + j); System.out.println(" x = " + x); System.out.println(" y = " + y); //adding numbers System.out.println("Adding..."); System.out.println(" i + j = " + (i + j)); System.out.println(" x + y = " + (x + y)); //subtracting numbers System.out.println("Subtracting..."); System.out.println(" i - j = " + (i - j)); System.out.println(" x - y = " + (x - y)); //multiplying numbers System.out.println("Multiplying..."); System.out.println(" i * j = " + (i * j)); System.out.println(" x * y = " + (x * y)); //dividing numbers System.out.println("Dividing..."); System.out.println(" i / j = " + (i / j)); System.out.println(" x / y = " + (x / y)); //computing the remainder resulting from dividing numbers System.out.println("Computing the remainder..."); System.out.println(" i % j = " + (i % j)); System.out.println(" x % y = " + (x % y)); //mixing types System.out.println("Mixing types..."); System.out.println(" j + y = " + (j + y)); System.out.println(" i * x = " + (i * x)); } } Introduction to Programming I 71
- 72. J.E.D.I Here is the output of the program, Variable values... i = 37 j = 42 x = 27.475 y = 7.22 Adding... i + j = 79 x + y = 34.695 Subtracting... i - j = -5 x - y = 20.255 Multiplying... i * j = 1554 x * y = 198.37 Dividing... i / j = 0 x / y = 3.8054 Computing the remainder... i % j = 37 x % y = 5.815 Mixing types... j + y = 49.22 i * x = 1016.58 Note: When an integer and a floating-point number are used as operands to a single arithmetic operation, the result is a floating point. The integer is implicitly converted to a floating-point number before the operation takes place. Introduction to Programming I 72
- 73. J.E.D.I 4.10.2 Increment and Decrement operators Aside from the basic arithmetic operators, Java also includes a unary increment operator (++) and unary decrement operator (--). Increment and decrement operators increase and decrease a value stored in a number variable by 1. For example, the expression, count = count + 1; //increment the value of count by 1 is equivalent to, count++; Operator Use Description ++ op++ Increments op by 1; evaluates to the value of op before it was incremented ++ ++op Increments op by 1; evaluates to the value of op after it was incremented -- op-- Decrements op by 1; evaluates to the value of op before it was decremented -- --op Decrements op by 1; evaluates to the value of op after it was decremented Table 11: Increment and Decrement operators The increment and decrement operators can be placed before or after an operand. When used before an operand, it causes the variable to be incremented or decremented by 1, and then the new value is used in the expression in which it appears. For example, int i = 10, int j = 3; int k = 0; k = ++j + i; //will result to k = 4+10 = 14 Introduction to Programming I 73
- 74. J.E.D.I When the increment and decrement operators are placed after the operand, the old value of the variable will be used in the expression where it appears. For example, int i = 10, int j = 3; int k = 0; k = j++ + i; //will result to k = 3+10 = 13 Coding Guideline: Always keep expressions containing increment and decrement operators simple and easy to understand. Introduction to Programming I 74
- 75. J.E.D.I 4.10.3 Relational operators Relational operators compare two values and determines the relationship between those values. The output of evaluation are the boolean values true or false. Operator Use Description > op1 > op2 op1 is greater than op2 >= op1 >= op2 op1 is greater than or equal to op2 < op1 < op2 op1 is less than op2 <= op1 <= op2 op1 is less than or equal to op2 == op1 == op2 op1 and op2 are equal != op1 != op2 op1 and op2 are not equal Table 12: Relational Operators Introduction to Programming I 75
- 76. J.E.D.I Here's a sample program that uses relational operators, public class RelationalDemo { public static void main(String[] args) { //a few numbers int i = 37; int j = 42; int k = 42; System.out.println("Variable values..."); System.out.println(" i = " + i); System.out.println(" j = " + j); System.out.println(" k = " + k); //greater than System.out.println("Greater than..."); System.out.println(" i > j = " + (i > j)); //false System.out.println(" j > i = " + (j > i)); //true System.out.println(" k > j = " + (k > j)); //false //greater than or equal to System.out.println("Greater than or equal to..."); System.out.println(" i >= j = " + (i >= j)); //false System.out.println(" j >= i = " + (j >= i)); //true System.out.println(" k >= j = " + (k >= j)); //true //less than System.out.println("Less than..."); System.out.println(" i < j = " + (i < j)); //true System.out.println(" j < i = " + (j < i)); //false System.out.println(" k < j = " + (k < j)); //false //less than or equal to System.out.println("Less than or equal to..."); System.out.println(" i <= j = " + (i <= j)); //true System.out.println(" j <= i = " + (j <= i)); //false System.out.println(" k <= j = " + (k <= j)); //true //equal to System.out.println("Equal to..."); System.out.println(" i == j = " + (i == j)); //false System.out.println(" k == j = " + (k == j)); //true //not equal to System.out.println("Not equal to..."); System.out.println(" i != j = " + (i != j)); //true System.out.println(" k != j = " + (k != j)); //false } } Introduction to Programming I 76
- 77. J.E.D.I Here's the output from this program: Variable values... i = 37 j = 42 k = 42 Greater than... i > j = false j > i = true k > j = false Greater than or equal to... i >= j = false j >= i = true k >= j = true Less than... i < j = true j < i = false k < j = false Less than or equal to... i <= j = true j <= i = false k <= j = true Equal to... i == j = false k == j = true Not equal to... i != j = true k != j = false Introduction to Programming I 77
- 78. J.E.D.I 4.10.4 Logical operators Logical operators have one or two boolean operands that yield a boolean result. There are six logical operators: && (logical AND), & (boolean logical AND), || (logical OR), | (boolean logical inclusive OR), ^ (boolean logical exclusive OR), and ! (logical NOT). The basic expression for a logical operation is, x1 op x2 where x1, x2 can be boolean expressions, variables or constants, and op is either &&, &, ||, | or ^ operator. The truth tables that will be shown next, summarize the result of each operation for all possible combinations of x1 and x2. Introduction to Programming I 78
- 79. J.E.D.I 4.10.4.1 && (logical AND) and & (boolean logical AND) Here is the truth table for && and &, x1 x2 Result TRUE TRUE TRUE TRUE FALSE FALSE FALSE TRUE FALSE FALSE FALSE FALSE Table 13: Truth table for & and && The basic difference between && and & operators is that && supports short-circuit evaluations (or partial evaluations), while & doesn't. What does this mean? Given an expression, exp1 && exp2 && will evaluate the expression exp1, and immediately return a false value is exp1 is false. If exp1 is false, the operator never evaluates exp2 because the result of the operator will be false regardless of the value of exp2. In contrast, the & operator always evaluates both exp1 and exp2 before returning an answer. Here's a sample source code that uses logical and boolean AND, public class TestAND { public static void main( String[] args ){ int i = 0; int j = 10; boolean test= false; //demonstrate && test = (i > 10) && (j++ > 9); System.out.println(i); System.out.println(j); System.out.println(test); //demonstrate & test = (i > 10) & (j++ > 9); System.out.println(i); System.out.println(j); System.out.println(test); } } Introduction to Programming I 79
- 80. J.E.D.I The output of the program is, 0 10 false 0 11 false Note, that the j++ on the line containing the && operator is not evaluated since the first expression (i>10) is already equal to false. Introduction to Programming I 80
- 81. J.E.D.I 4.10.4.2 || (logical OR) and | (boolean logical inclusive OR) Here is the truth table for || and |, x1 x2 Result TRUE TRUE TRUE TRUE FALSE TRUE FALSE TRUE TRUE FALSE FALSE FALSE Table 14: Truth table for | and || The basic difference between || and | operators is that || supports short-circuit evaluations (or partial evaluations), while | doesn't. What does this mean? Given an expression, exp1 || exp2 || will evaluate the expression exp1, and immediately return a true value is exp1 is true. If exp1 is true, the operator never evaluates exp2 because the result of the operator will be true regardless of the value of exp2. In contrast, the | operator always evaluates both exp1 and exp2 before returning an answer. Here's a sample source code that uses logical and boolean OR, public class TestOR { public static void main( String[] args ){ int i = 0; int j = 10; boolean test= false; //demonstrate || test = (i < 10) || (j++ > 9); System.out.println(i); System.out.println(j); System.out.println(test); //demonstrate | test = (i < 10) | (j++ > 9); System.out.println(i); System.out.println(j); System.out.println(test); } } Introduction to Programming I 81
- 82. J.E.D.I The output of the program is, 0 10 true 0 11 true Note, that the j++ on the line containing the || operator is not evaluated since the first expression (i<10) is already equal to true. Introduction to Programming I 82
- 83. J.E.D.I 4.10.4.3 ^ (boolean logical exclusive OR) Here is the truth table for ^, x1 x2 Result TRUE TRUE FALSE TRUE FALSE TRUE FALSE TRUE TRUE FALSE FALSE FALSE Table 15: Truth table for ^ The result of an exclusive OR operation is TRUE, if and only if one operand is true and the other is false. Note that both operands must always be evaluated in order to calculate the result of an exclusive OR. Here's a sample source code that uses the logical exclusive OR operator, public class TestXOR { public static void main( String[] args ){ boolean val1 = true; boolean val2 = true; System.out.println(val1 ^ val2); val1 = false; val2 = true; System.out.println(val1 ^ val2); val1 = false; val2 = false; System.out.println(val1 ^ val2); val1 = true; val2 = false; System.out.println(val1 ^ val2); } } The output of the program is, false true false true Introduction to Programming I 83
- 84. J.E.D.I 4.10.4.4 ! (logical NOT) The logical NOT takes in one argument, wherein that argument can be an expression, variable or constant. Here is the truth table for !, x1 Result TRUE FALSE FALSE TRUE Table 16: Truth table for ! Here's a sample source code that uses the logical NOT operator, public class TestNOT { public static void main( String[] args ){ boolean val1 = true; boolean val2 = false; System.out.println(!val1); System.out.println(!val2); } } The output of the program is, false true Introduction to Programming I 84
- 85. J.E.D.I 4.10.5 Conditional Operator (?:) The conditional operator ?: is a ternary operator. This means that it takes in three arguments that together form a conditional expression. The structure of an expression using a conditional operator is, exp1?exp2:exp3 wherein exp1 is a boolean expression whose result must either be true or false. If exp1 is true, exp2 is the value returned. If it is false, then exp3 is returned. For example, given the code, public class ConditionalOperator { public static void main( String[] args ){ String status = ""; int grade = 80; //get status of the student status = (grade >= 60)?"Passed":"Fail"; //print status System.out.println( status ); } } The output of this program will be, Passed Introduction to Programming I 85
- 86. J.E.D.I Here is the flowchart of how ?: works, Here is another program that uses the ?: operator, class ConditionalOperator { public static void main( String[] args ){ int score = 0; char answer = 'a'; score = (answer == 'a') ? 10 : 0; System.out.println("Score = " + score ); } } The output of the program is, Score = 10 Introduction to Programming I 86 Figure 4.2: Flowchart using the ?: operator
- 87. J.E.D.I 4.10.6 Operator Precedence Operator precedence defines the compiler’s order of evaluation of operators so as to come up with an unambiguous result. Given a complicated expression, 6%2*5+4/2+88-10 we can re-write the expression and place some parenthesis base on operator precedence, ((6%2)*5)+(4/2)+88-10; Coding Guidelines To avoid confusion in evaluating mathematical operations, keep your expressions simple and use parenthesis. Introduction to Programming I 87 Figure 4.3: Operator Precedence
- 88. J.E.D.I 4.11 Exercises 4.11.1 Declaring and printing variables Given the table below, declare the following variables with the corresponding data types and initialization values. Output to the screen the variable names together with the values. Variable name Data Type Initial value number integer 10 letter character a result boolean true str String hello The following should be the expected screen output, Number = 10 letter = a result = true str = hello 4.11.2 Getting the average of three numbers Create a program that outputs the average of three numbers. Let the values of the three numbers be, 10, 20 and 45. The expected screen output is, number 1 = 10 number 2 = 20 number 3 = 45 Average is = 25 4.11.3 Output greatest value Given three numbers, write a program that outputs the number with the greatest value among the three. Use the conditional ?: operator that we have studied so far (HINT: You will need to use two sets of ?: to solve this). For example, given the numbers 10, 23 and 5, your program should output, number 1 = 10 number 2 = 23 number 3 = 5 The highest number is = 23 4.11.4 Operator precedence Given the following expressions, re-write them by writing some parenthesis based on the sequence on how they will be evaluated. 1. a / b ^ c ^ d – e + f – g * h + i 2. 3 * 10 *2 / 15 – 2 + 4 ^ 2 ^ 2 3. r ^ s * t / u – v + w ^ x – y++ Introduction to Programming I 88
- 89. J.E.D.I 5 Getting Input from the Keyboard 5.1 Objectives Now that we've studied some basic concepts in Java and we've written some simple programs, let's make our programs more interactive by getting some input from the user. In this section, we'll be discussing two methods of getting input, the first one is through the use of the BufferedReader class and the other one involves a graphical user interface by using JOptionPane. At the end of the lesson, the student should be able to: • Create an interactive Java program that gets input from the keyboard • Use the BufferedReader class to get input from the keyboard using a console • Use the JOptionPane class to get input from the keyboard using a graphical user interface 5.2 Using BufferedReader to get input In this section, we will use the BufferedReader class found in the java.io package in order to get input from the keyboard. Here are the steps to get input from the keyboard: 1. Add this at the top of your code: import java.io.*; 2. Add this statement: BufferedReader dataIn = new BufferedReader( new InputStreamReader( System.in) ); 3. Declare a temporary String variable to get the input, and invoke the readLine() method to get input from the keyboard. You have to type it inside a try-catch block. try{ String temp = dataIn.readLine(); } catch( IOException e ){ System.out.println(“Error in getting input”); } Introduction to Programming I 89
- 90. J.E.D.I Here is the complete source code: import java.io.BufferedReader; import java.io.InputStreamReader; import java.io.IOException; public class GetInputFromKeyboard { public static void main( String[] args ){ BufferedReader dataIn = new BufferedReader(new InputStreamReader( System.in) ); String name = ""; System.out.print("Please Enter Your Name:"); try{ name = dataIn.readLine(); }catch( IOException e ){ System.out.println("Error!"); } System.out.println("Hello " + name +"!"); } } Now let's try to explain each line of code: The statements, import java.io.BufferedReader; import java.io.InputStreamReader; import java.io.IOException; indicate that we want to use the classes BufferedReader, InputStreamReader and IOException which is inside the java.io package. The Java Application Programming Interface (API) contains hundreds of predefined classes that you can use in your programs. These classes are organized into what we call packages. Packages contain classes that have related purpose. Just like in our example, the java.io package contains classes that allow programs to input and output data. The statements can also be rewritten as, import java.io.*; which will load all the classes found in the package, and then we can use those classes inside our program. Introduction to Programming I 90
- 91. J.E.D.I The next two statements, public class GetInputFromKeyboard { public static void main( String[] args ){ were already discussed in the previous lesson. This means we declare a class named GetInputFromKeyboard and we declare the main method. In the statement, BufferedReader dataIn = new BufferedReader(new InputStreamReader( System.in) ); we are declaring a variable named dataIn with the class type BufferedReader. Don't worry about what the syntax means for now. We will cover more about this later in the course. Now, we are declaring a String variable with the identifier name, String name = ""; This is where we will store the input of the user. The variable name is initialized to an empty String "". It is always good to initialize your variables as you declare them. The next line just outputs a String on the screen asking for the user's name. System.out.print("Please Enter Your Name:"); Now, the following block defines a try-catch block, try{ name = dataIn.readLine(); }catch( IOException e ){ System.out.println("Error!"); } This assures that the possible exceptions that could occur in the statement name = dataIn.readLine(); will be catched. We will cover more about exception handling in the latter part of this course, but for now, just take note that you need to add this code in order to use the readLine() method of BufferedReader to get input from the user. Introduction to Programming I 91
- 92. J.E.D.I Now going back to the statement, name = dataIn.readLine(); the method call, dataIn.readLine(), gets input from the user and will return a String value. This value will then be saved to our name variable, which we will use in our final statement to greet the user, System.out.println("Hello " + name + "!"); Introduction to Programming I 92
- 93. J.E.D.I 5.3 Using JOptionPane to get input Another way to get input from the user is by using the JOptionPane class which is found in the javax.swing package. JOptionPane makes it easy to pop up a standard dialog box that prompts users for a value or informs them of something. Given the following code, import javax.swing.JOptionPane; public class GetInputFromKeyboard { public static void main( String[] args ){ String name = ""; name = JoptionPane.showInputDialog("Please enter your name"); String msg = "Hello " + name + "!"; JOptionPane.showMessageDialog(null, msg); } } This will output, Introduction to Programming I 93 Figure 5.1: Getting Input Using JOptionPane Figure 5.2: Input florence on the JOptionPane Figure 5.3: Showing Message Using JOptionPane
- 94. J.E.D.I The first statement, import javax.swing.JOptionPane; indicates that we want to import the class JOptionPane from the javax.swing package. We can also write this as, import javax.swing.*; The statement, name = JOptionPane.showInputDialog("Please enter your name"); creates a JOptionPane input dialog, which will display a dialog with a message, a textfield and an OK button as shown in the figure. This returns a String which we will save in the name variable. Now we create the welcome message, which we will store in the msg variable, String msg = "Hello " + name + "!"; The next line displays a dialog which contains a message and an OK button. JOptionPane.showMessageDialog(null, msg); Introduction to Programming I 94
- 95. J.E.D.I 5.4 Exercises 5.4.1 Last 3 words (BufferedReader version) Using BufferedReader, ask for three words from the user and output those three words on the screen. For example, Enter word1:Goodbye Enter word2:and Enter word3:Hello Goodbye and Hello 5.4.2 Last 3 words (JOptionPane version) Using JOptionPane, ask for three words from the user and output those three words on the screen. For example, Introduction to Programming I 95 Figure 5.4: First Input Figure 5.5: Second Input Figure 5.6: Third Input Figure 5.7: Show Message
- 96. J.E.D.I 6 Control Structures 6.1 Objectives In the previous sections, we have given examples of sequential programs, wherein statements are executed one after another in a fixed order. In this section, we will be discussing control structures, which allows us to change the ordering of how the statements in our programs are executed. At the end of the lesson, the student should be able to: • Use decision control structures (if, else, switch) which allows selection of specific sections of code to be executed • Use repetition control structures (while, do-while, for) which allow executing specific sections of code a number of times • Use branching statements (break, continue, return) which allows redirection of program flow 6.2 Decision Control Structures Decision control structures are Java statements that allows us to select and execute specific blocks of code while skipping other sections. 6.2.1 if statement The if-statement specifies that a statement (or block of code) will be executed if and only if a certain boolean statement is true. The if-statement has the form, if( boolean_expression ) statement; or if( boolean_expression ){ statement1; statement2; . . . } where, boolean_expression is either a boolean expression or boolean variable. Introduction to Programming I 96
- 97. J.E.D.I if( grade > 60 ) System.out.println("Congratulations!"); or int grade = 68; if( grade > 60 ){ System.out.println("Congratulations!"); System.out.println("You passed!"); } Coding Guidelines: 1. The boolean_expression part of a statement should evaluate to a boolean value. That means that the execution of the condition should either result to a value of true or a false. 2. Indent the statements inside the if-block.For example, if( boolean_expression ){ //statement1; //statement2; } Introduction to Programming I 97 Figure 6.1: Flowchart of If-Statement
- 98. J.E.D.I 6.2.2 if-else statement The if-else statement is used when we want to execute a certain statement if a condition is true, and a different statement if the condition is false. The if-else statement has the form, if( boolean_expression ) statement; else statement; or can also be written as, if( boolean_expression ){ statement1; statement2; . . . } else{ statement1; statement2; . . . } For example, given the code snippet, int grade = 68; if( grade > 60 ) System.out.println("Congratulations!"); else System.out.println("Sorry you failed"); or int grade = 68; if( grade > 60 ){ System.out.println("Congratulations!"); System.out.println("You passed!"); } else{ System.out.println("Sorry you failed"); } Introduction to Programming I 98
- 99. J.E.D.I Coding Guidelines: 1. To avoid confusion, always place the statement or statements of an if or if-else block inside brackets {}. 2. You can have nested if-else blocks. This means that you can have other if-else blocks inside another if-else block.For example, if( boolean_expression ){ if( boolean_expression ){ . . . } } else{ . . . } Introduction to Programming I 99 Figure 6.2: Flowchart of If-Else Statement
- 100. J.E.D.I 6.2.3 if-else-if statement The statement in the else-clause of an if-else block can be another if-else structures. This cascading of structures allows us to make more complex selections. The if-else if statement has the form, if( boolean_expression1 ) statement1; else if( boolean_expression2 ) statement2; else statement3; Take note that you can have many else-if blocks after an if-statement. The else-block is optional and can be omitted. In the example shown above, if boolean_expression1 is true, then the program executes statement1 and skips the other statements. If boolean_expression2 is true, then the program executes statement 2 and skips to the statements following statement3. Introduction to Programming I 100 Figure 6.3: Flowchart of If-Else-If Statement
- 101. J.E.D.I For example, given the code snippet, int grade = 68; if( grade > 90 ){ System.out.println("Very good!"); } else if( grade > 60 ){ System.out.println("Very good!"); } else{ System.out.println("Sorry you failed"); } 6.2.4 Common Errors when using the if-else statements: 1. The condition inside the if-statement does not evaluate to a boolean value. For example, //WRONG int number = 0; if( number ){ //some statements here } The variable number does not hold a Boolean value. 2. Using = instead of == for comparison. For example, //WRONG int number = 0; if( number = 0 ){ //some statements here } This should be written as, //CORRECT int number = 0; if( number == 0 ){ //some statements here } 3. Writing elseif instead of else if. Introduction to Programming I 101
- 102. J.E.D.I 6.2.5 Example for if-else-else if public class Grade { public static void main( String[] args ) { double grade = 92.0; if( grade >= 90 ){ System.out.println( "Excellent!" ); } else if( (grade < 90) && (grade >= 80)){ System.out.println("Good job!" ); } else if( (grade < 80) && (grade >= 60)){ System.out.println("Study harder!" ); } else{ System.out.println("Sorry, you failed."); } } } Introduction to Programming I 102
- 103. J.E.D.I 6.2.6 switch statement Another way to indicate a branch is through the switch keyword. The switch construct allows branching on multiple outcomes. The switch statement has the form, switch( switch_expression ){ case case_selector1: statement1; // statement2; //block 1 . . . // break; case case_selector2: statement1; // statement2; //block 2 . . . // break; . . . default: statement1; // statement2; //block n . . . // break; } where, switch_expression is an integer or character expression and, case_selector1, case_selector2 and so on, are unique integer or character constants. When a switch is encountered, Java first evaluates the switch_expression, and jumps to the case whose selector matches the value of the expression. The program executes the statements in order from that point on until a break statement is encountered, skipping then to the first statement after the end of the switch structure. If none of the cases are satisfied, the default block is executed. Take note however, that the default part is optional. A switch statement can have no default block. NOTES: • Unlike with the if statement, the multiple statements are executed in the switch statement without needing the curly braces. • When a case in a switch statement has been matched, all the statements associated with that case are executed. Not only that, the statements associated with the succeeding cases are also executed. • To prevent the program from executing statements in the subsequent cases, we use a break statement as our last statement. Introduction to Programming I 103
- 104. J.E.D.I Coding Guidelines: 1. Deciding whether to use an if statement or a switch statement is a judgment call. You can decide which to use, based on readability and other factors. 2. An if statement can be used to make decisions based on ranges of values or conditions, whereas a switch statement can make decisions based only on a single integer or character value. Also, the value provided to each case statement must be unique. Introduction to Programming I 104 Figure 6.4: Flowchart of Switch Statements
- 105. J.E.D.I 6.2.7 Example for switch public class Grade { public static void main( String[] args ) { int grade = 92; switch(grade){ case 100: System.out.println( "Excellent!" ); break; case 90: System.out.println("Good job!" ); break; case 80: System.out.println("Study harder!" ); break; default: System.out.println("Sorry, you failed."); } } } Introduction to Programming I 105
- 106. J.E.D.I 6.3 Repetition Control Structures Repetition control structures are Java statements that allows us to execute specific blocks of code a number of times. There are three types of repetition control structures, the while, do-while and for loops. 6.3.1 while loop The while loop is a statement or block of statements that is repeated as long as some condition is satisfied. The while statement has the form, while( boolean_expression ){ statement1; statement2; . . . } The statements inside the while loop are executed as long as the boolean_expression evaluates to true. For example, given the code snippet, int i = 4; while ( i > 0 ){ System.out.print(i); i--; } The sample code shown will print 4321 on the screen. Take note that if the line containing the statement i--; is removed, this will result to an infinite loop, or a loop that does not terminate. Therefore, when using while loops or any kind of repetition control structures, make sure that you add some statements that will allow your loop to terminate at some point. Introduction to Programming I 106
- 107. J.E.D.I The following are other examples of while loops, Example 1: int x = 0; while (x<10) { System.out.println(x); x++; } Example 2: //infinite loop while(true) System.out.println(“hello”); Example 3: //no loops // statement is not even executed while (false) System.out.println(“hello”); Introduction to Programming I 107
- 108. J.E.D.I 6.3.2 do-while loop The do-while loop is similar to the while-loop. The statements inside a do-while loop are executed several times as long as the condition is satisfied. The main difference between a while and do-while loop is that, the statements inside a do-while loop are executed at least once. The do-while statement has the form, do{ statement1; statement2; . . . }while( boolean_expression ); The statements inside the do-while loop are first executed, and then the condition in the boolean_expression part is evaluated. If this evaluates to true, the statements inside the do-while loop are executed again. Here are a few examples that uses the do-while loop: Example 1: int x = 0; do { System.out.println(x); x++; }while (x<10); This example will output 0123456789 on the screen. Example 2: //infinite loop do{ System.out.println(“hello”); } while (true); This example will result to an infinite loop, that prints hello on screen. Example 3: //one loop // statement is executed once do System.out.println(“hello”); while (false); This example will output hello on the screen. Introduction to Programming I 108
- 109. J.E.D.I Coding Guidelines: 1. Common programming mistakes when using the do-while loop is forgetting to write the semi-colon after the while expression. do{ ... }while(boolean_expression) //WRONG->forgot semicolon ; 2. Just like in while loops, make sure that your do-while loops will terminate at some point. 6.3.3 for loop The for loop, like the previous loops, allows execution of the same code a number of times. The for loop has the form, for (InitializationExpression; LoopCondition; StepExpression){ statement1; statement2; . . . } where, InitializationExpression -initializes the loop variable. LoopCondition - compares the loop variable to some limit value. StepExpression - updates the loop variable. A simple example of the for loop is, int i; for( i = 0; i < 10; i++ ){ System.out.print(i); } In this example, the statement i=0, first initializes our variable. After that, the condition expression i<10 is evaluated. If this evaluates to true, then the statement inside the for loop is executed. Next, the expression i++ is executed, and then the condition expression is again evaluated. This goes on and on, until the condition expression evaluates to false. This example, is equivalent to the while loop shown below, int i = 0; while( i < 10 ){ System.out.print(i); i++; } Introduction to Programming I 109
- 110. J.E.D.I 6.4 Branching Statements Branching statements allows us to redirect the flow of program execution. Java offers three branching statements: break, continue and return. 6.4.1 break statement The break statement has two forms: unlabeled (we saw its unlabeled form in the switch statement) and labeled. 6.4.1.1 Unlabeled break statement The unlabeled break terminates the enclosing switch statement, and flow of control transfers to the statement immediately following the switch. You can also use the unlabeled form of the break statement to terminate a for, while, or do-while loop. For example, String names[] = {"Beah", "Bianca", "Lance", "Belle", "Nico", "Yza", "Gem", "Ethan"}; String searchName = "Yza"; boolean foundName = false; for( int i=0; i< names.length; i++ ){ if( names[i].equals( searchName )){ foundName = true; break; } } if( foundName ){ System.out.println( searchName + " found!" ); } else{ System.out.println( searchName + " not found." ); } In this example, if the search string "Yza" is found, the for loop will stop and flow of control transfers to the statement following the for loop. Introduction to Programming I 110
- 111. J.E.D.I 6.4.1.2 Labeled break statement The labeled form of a break statement terminates an outer statement, which is identified by the label specified in the break statement. The following program searches for a value in a two-dimensional array. Two nested for loops traverse the array. When the value is found, a labeled break terminates the statement labeled search, which is the outer for loop. int[][] numbers = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; int searchNum = 5; boolean foundNum = false; searchLabel: for( int i=0; i<numbers.length; i++ ){ for( int j=0; j<numbers[i].length; j++ ){ if( searchNum == numbers[i][j] ){ foundNum = true; break searchLabel; } } } if( foundNum ){ System.out.println( searchNum + " found!" ); } else{ System.out.println( searchNum + " not found!" ); } The break statement terminates the labeled statement; it does not transfer the flow of control to the label. The flow of control transfers to the statement immediately following the labeled (terminated) statement. Introduction to Programming I 111
- 112. J.E.D.I 6.4.2 continue statement The continue statement has two forms: unlabeled and labeled. You can use the continue statement to skip the current iteration of a for, while or do-while loop. 6.4.2.1 Unlabeled continue statement The unlabeled form skips to the end of the innermost loop's body and evaluates the boolean expression that controls the loop, basically skipping the remainder of this iteration of the loop. The following example counts the number of "Beah"s in the array. String names[] = {"Beah", "Bianca", "Lance", "Beah"}; int count = 0; for( int i=0; i<names.length; i++ ){ if( !names[i].equals("Beah") ){ continue; //skip next statement } count++; } System.out.println("There are " + count + " Beahs in the list"); 6.4.2.2 Labeled continue statement The labeled form of the continue statement skips the current iteration of an outer loop marked with the given label. outerLoop: for( int i=0; i<5; i++ ){ for( int j=0; j<5; j++ ){ System.out.println("Inside for(j) loop"); //message1 if( j == 2 ) continue outerLoop; } System.out.println("Inside for(i) loop"); //message2 } In this example, message 2 never gets printed since we have the statement continue outerloop which skips the iteration. Introduction to Programming I 112
- 113. J.E.D.I 6.4.3 return statement The return statement is used to exit from the current method. The flow of control returns to the statement that follows the original method call. The return statement has two forms: one that returns a value and one that doesn't. To return a value, simply put the value (or an expression that calculates the value) after the return keyword. For example, return ++count; or return "Hello"; The data type of the value returned by return must match the type of the method's declared return value. When a method is declared void, use the form of return that doesn't return a value. For example, return; We will cover more about return statements later when we discuss about methods. Introduction to Programming I 113
- 114. J.E.D.I 6.5 Exercises 6.5.1 Grades Get three exam grades from the user and compute the average of the grades. Output the average of the three exams. Together with the average, also include a smiley face in the output if the average is greater than or equal to 60, otherwise output :-(. 1. Use BufferedReader to get input from the user, and System.out to output the result. 2. Use JOptionPane to get input from the user and to output the result. 6.5.2 Number in words Get a number as input from the user, and output the equivalent of the number in words. The number inputted should range from 1-10. If the user inputs a number that is not in the range, output, "Invalid number". 1. Use an if-else statement to solve this problem 2. Use a switch statement to solve this problem 6.5.3 Hundred Times Create a program that prints your name a hundred times. Do three versions of this program using a while loop, a do-while loop and a for-loop. 6.5.4 Powers Compute the power of a number given the base and exponent. Do three versions of this program using a while loop, a do-while loop and a for-loop. Introduction to Programming I 114
- 115. J.E.D.I 7 Java Arrays 7.1 Objectives In this section, we will be discussing about Java Arrays. First, we are going to define what arrays are, and then we are going to discuss on how to declare and use them. At the end of the lesson, the student should be able to: • Declare and create arrays • Access array elements • Determine the number of elements in an array • Declare and create multidimensional arrays 7.2 Introduction to arrays In the previous sections, we have discussed on how to declare different variables using the primitive data types. In declaring variables, we often use a unique identifier or name and a datatype. In order to use the variable, we call it by its identifier name. For example, we have here three variables of type int with different identifiers for each variable. int number1; int number2; int number3; number1 = 1; number2 = 2; number3 = 3; As you can see, it seems like a tedious task in order to just initialize and use the variables especially if they are used for the same purpose. In Java and other programming languages, there is one capability wherein we can use one variable to store a list of data and manipulate them more efficiently. This type of variable is called an array. An array stores multiple data items of the same datatype, in a contiguous block of memory, divided into a number of slots. Think of an array as a stretched variable – a location that still has one identifier name, but can hold more than one value. Introduction to Programming I 115 Figure 7.1: Example of an Integer Array
- 116. J.E.D.I 7.3 Declaring Arrays Arrays must be declared like all variables. When declaring an array, you list the data type, followed by a set of square brackets[], followed by the identifier name. For example, int []ages; or you can place the brackets after the identifier. For example, int ages[]; After declaring, we must create the array and specify its length with a constructor statement. This process in Java is called instantiation (the Java word for creates). In order to instantiate an object, we need to use a constructor for this. We will cover more about instantiating objects and constructors later. Take note, that the size of an array cannot be changed once you've initialized it. For example, //declaration int ages[]; //instantiate object ages = new int[100]; or, can also be written as, //declare and instantiate object int ages[] = new int[100]; In the example, the declaration tells the Java Compiler that the identifier ages will be used as the name of an array containing integers, and to create or instantiate a new array containing 100 elements. Instead of using the new keyword to instantiate an array, you can also automatically declare, construct and assign values at once. Introduction to Programming I 116 Figure 7.2: Instantiating Arrays
- 117. J.E.D.I Examples are, //creates an array of boolean variables with ientifier //results. This array contains 4 elements that are //initialized to values {true, false, true, false} boolean results[] ={ true, false, true, false }; //creates an array of 4 double variables initialized //to the values {100, 90, 80, 75}; double []grades = {100, 90, 80, 75}; //creates an array of Strings with identifier days and //initialized. This array contains 7 elements String days[] = { “Mon”, “Tue”, “Wed”, “Thu”, “Fri”, “Sat”, “Sun”}; Introduction to Programming I 117
- 118. J.E.D.I 7.4 Accessing an array element To access an array element, or a part of the array, you use a number called an index or a subscript. An index number or subscript is assigned to each member of the array, allowing the program and the programmer to access individual values when necessary. Index numbers are always integers. They begin with zero and progress sequentially by whole numbers to the end of the array. Take note that the elements inside your array is from 0 to (sizeOfArray-1). For example, given the array we declared a while ago, we have //assigns 10 to the first element in the array ages[0] = 10; //prints the last element in the array System.out.print(ages[99]); Take note that once an array is declared and constructed, the stored value of each member of the array will be initialized to zero for number data. However, reference data types such as Strings are not initialized to blanks or an empty string “”. Therefore, you must populate the String arrays explicitly. The following is a sample code on how to print all the elements in the array. This uses a for loop, so our code is shorter. public class ArraySample{ public static void main( String[] args ){ int[] ages = new int[100]; for( int i=0; i<100; i++ ){ System.out.print( ages[i] ); } } } Coding Guidelines: 1. It is usually better to initialize or instantiate the array right away after you declare it. For example, the declaration, int []arr = new int[100]; is preferred over, int []arr; arr = new int[100]; 2. The elements of an n-element array have indexes from 0 to n-1. Note that there is no array element arr[n]! This will result in an array-index-out-of-bounds exception. 3. You cannot resize an array. Introduction to Programming I 118
- 119. J.E.D.I 7.5 Array length In order to get the number of elements in an array, you can use the length field of an array. The length field of an array returns the size of the array. It can be used by writing, arrayName.length For example, given the previous example, we can re-write it as, public class ArraySample { public static void main( String[] args ){ int[] ages = new int[100]; for( int i=0; i<ages.length; i++ ){ System.out.print( ages[i] ); } } } Coding Guidelines: 1. When creating for loops to process the elements of an array, use the array object's length field in the condition statement of the for loop. This will allow the loop to adjust automatically for different-sized arrays. 2. Declare the sizes of arrays in a Java program using named constants to make them easy to change. For example, final int ARRAY_SIZE = 1000; //declare a constant . . . int[] ages = new int[ARRAY_SIZE]; Introduction to Programming I 119
- 120. J.E.D.I 7.6 Multidimensional Arrays Multidimensional arrays are implemented as arrays of arrays. Multidimensional arrays are declared by appending the appropriate number of bracket pairs after the array name. For example, // integer array 512 x 128 elements int[][] twoD = new int[512][128]; // character array 8 x 16 x 24 char[][][] threeD = new char[8][16][24]; // String array 4 rows x 2 columns String[][] dogs = {{ "terry", "brown" }, { "Kristin", "white" }, { "toby", "gray"}, { "fido", "black"} }; To access an element in a multidimensional array is just the same as accessing the elements in a one dimensional array. For example, to access the first element in the first row of the array dogs, we write, System.out.print( dogs[0][0] ); This will print the String "terry" on the screen. Introduction to Programming I 120
- 121. J.E.D.I 7.7 Exercises 7.7.1 Days of the Week Create an array of Strings which are initialized to the 7 days of the week. For Example, String days[] = {“Monday”, “Tuesday”….}; Using a while-loop, print all the contents of the array. (do the same for do-while and for- loop) 7.7.2 Greatest number Using BufferedReader or JOptionPane, ask for 10 numbers from the user. Use an array to store the values of these 10 numbers. Output on the screen the number with the greatest value. 7.7.3 Addressbook Entries Given the following multidimensional array that contains addressbook entries: String entry = {{"Florence", "735-1234", "Manila"}, {"Joyce", "983-3333", "Quezon City"}, {"Becca", "456-3322", "Manila"}}; Print the following entries on screen in the following format: Name : Florence Tel. # : 735-1234 Address : Manila Name : Joyce Tel. # : 983-3333 Address : Quezon City Name : Becca Tel. # : 456-3322 Address : Manila Introduction to Programming I 121
- 122. J.E.D.I 8 Command-line Arguments 8.1 Objectives In this section, we will study on how to process input from the command-line by using arguments pass onto a Java program. At the end of the lesson, the student should be able to: • Know and explain what a command-line argument is • Get input from the user using command-line arguments • Learn how to pass arguments to your programs in NetBeans 8.2 Command-line arguments A Java application can accept any number of arguments from the command-line. Command-line arguments allow the user to affect the operation of an application for one invocation. The user enters command-line arguments when invoking the application and specifies them after the name of the class to run. For example, suppose you have a Java application, called Sort, that sorts five numbers, you run it like this: Take note that the arguments are separated by spaces. Introduction to Programming I 122 Figure 8.1: Running with Command-Line Arguments
- 123. J.E.D.I In the Java language, when you invoke an application, the runtime system passes the command-line arguments to the application's main method via an array of Strings. Each String in the array contains one of the command-line arguments. Remember the declaration for the main method, public static void main( String[] args ) The arguments that are passed to your program are saved into an array of String with the args identifier. In the previous example, the command-line arguments passed to the Sort application is an array that contains five strings which are: "5", "4", "3", "2" and "1". You can derive the number of command-line arguments with the array's length attribute. For example, int numberOfArgs = args.length; If your program needs to support a numeric command-line argument, it must convert a String argument that represents a number, such as "34", to a number. Here's a code snippet that converts a command-line argument to an integer, int firstArg = 0; if (args.length > 0){ firstArg = Integer.parseInt(args[0]); } parseInt throws a NumberFormatException (ERROR) if the format of args[0] isn't valid (not a number). Coding Guidelines: Before using command-line arguments, always check if the number of arguments before accessing the array elements so that there will be no exception generated. Introduction to Programming I 123
- 124. J.E.D.I 8.3 Command-line arguments in NetBeans To illustrate on how to pass some arguments to your programs in NetBeans, let us create a Java program that will print the number of arguments and the first argument passed to it. public class CommandLineExample { public static void main( String[] args ){ System.out.println("Number of arguments=" + args.length); System.out.println("First Argument="+ args[0]); } } Now, run netbeans and create a new project and name this CommandLineExample. Copy the code shown above and compile the code. Now, follow these steps to pass arguments to your program using NetBeans. Click on Projects (encircled below). Introduction to Programming I 124 Figure 8.2: Opening Project File
- 125. J.E.D.I Right-click on the CommandLineExample icon, and a popup menu will appear. Click on Properties. The Project Properties dialog will then appear. Introduction to Programming I 125 Figure 8.3: Opening Properties Figure 8.4: Properties Dialog
- 126. J.E.D.I Now, click on Run-> Running Project. On the Arguments textbox, type the arguments you want to pass to your program. In this case we typed in the arguments 5 4 3 2 1. Then, click on the OK button. Introduction to Programming I 126 Figure 8.5: Click On Running Project Figure 8.6: Set the Command-Line Arguments
- 127. J.E.D.I Now try to RUN your program. As you can see here, the output to your program is the number of arguments which is 5, and the first argument which is 5. Introduction to Programming I 127 Figure 8.7: Running the Program in with the Shortcut Button Figure 8.8: Program Output
- 128. J.E.D.I 8.4 Exercises 8.4.1 Print arguments Get input from the user using command-line arguments and print all the arguments to the screen. For example, if the user entered, java Hello world that is all your program should print Hello world that is all 8.4.2 Arithmetic Operations Get two numbers from the user using command-line arguments and print sum, difference, product and quotient of the two numbers. For example, if the user entered, java ArithmeticOperation 20 4 your program should print sum = 24 difference = 16 product = 80 quotient = 5 Introduction to Programming I 128
- 129. J.E.D.I 9 Working with the Java Class Library 9.1 Objectives In this section, we will introduce some basic concepts of object-oriented programming. Later on, we will discuss the concept of classes and objects, and how to use these classes and their members. Comparison, conversion and casting of objects will also be covered. For now, we will focus on using classes that are already defined in the Java class library, we will discuss later on how to create your own classes. At the end of the lesson, the student should be able to: • Explain object-oriented programming and some of its concepts • Differentiate between classes and objects • Differentiate between instance variables/methods and class(static) variables/methods • Explain what methods are and how to call and pass parameters to methods • Identify the scope of a variable • Cast primitive data types and objects • Compare objects and determine the class of an objects 9.2 Introduction to Object-Oriented Programming Object-Oriented programming or OOP revolves around the concept of objects as the basic elements of your programs. When we compare this to the physical world, we can find many objects around us, such as cars, lion, people and so on. These objects are characterized by their properties (or attributes) and behaviors. For example, a car object has the properties, type of transmission, manufacturer and color. Its behaviors are turning, braking and accelerating. Similarly, we can define different properties and behavior of a lion. Please refer to the table below for the examples. Object Properties Behavior Car type of transmission manufacturer color turning braking accelerating Lion Weight Color hungry or not hungry tamed or wild roaring sleeping hunting Table 17: Example of Real-life Objects Introduction to Programming I 129
- 130. J.E.D.I With these descriptions, the objects in the physical world can easily be modeled as software objects using the properties as data and the behaviors as methods. These data and methods could even be used in programming games or interactive software to simulate the real-world objects! An example would be a car software object in a racing game or a lion software object in an educational interactive software zoo for kids. 9.3 Classes and Objects 9.3.1 Difference Between Classes and Objects In the software world, an object is a software component whose structure is similar to objects in the real world. Each object is composed of a set of data (properties/attributes) which are variables describing the essential characteristics of the object, and it also consists of a set of methods (behavior) that describes how an object behaves. Thus, an object is a software bundle of variables and related methods. The variables and methods in a Java object are formally known as instance variables and instance methods to distinguish them from class variables and class methods, which will be discussed later. The class is the fundamental structure in object-oriented programming. It can be thought of as a template, a prototype or a blueprint of an object. It consists of two types of members which are called fields (properties or attributes) and methods. Fields specifiy the data types defined by the class, while methods specify the operations. An object is an instance of the class. To differentiate between classes and objects, let us discuss an example. What we have here is a Car Class which can be used to define several Car Objects. In the table shown below, Car A and Car B are objects of the Car class. The class has fields plate number, color, manufacturer, and current speed which are filled-up with corresponding values in objects Car A and Car B. The Car has also some methods Accelerate, Turn and Brake. Car Class Object Car A Object Car B Instance Variables Plate Number ABC 111 XYZ 123 Color Blue Red Manufacturer Mitsubishi Toyota Current Speed 50 km/h 100 km/h Instance Methods Accelerate Method Turn Method Brake Method Table 18: Example of Car class and its objects When instantiated, each object gets a fresh set of state variables. However, the method implementations are shared among objects of the same class. Classes provide the benefit of reusability. Software programmers can use a class over and over again to create many objects. Introduction to Programming I 130
- 131. J.E.D.I 9.3.2 Encapsulation Encapsulation is the method of hiding certain elements of the implementation of a certain class. By placing a boundary around the properties and methods of our objects, we can prevent our programs from having side effects wherein programs have their variables changed in unexpected ways. We can prevent access to our object's data by declaring them declaring them in a certain way such that we can control access to them. We will learn more about how Java implements encapsulation as we discuss more about classes. 9.3.3 Class Variables and Methods In addition to the instance variables, it is also possible to define class variables, which are variables that belong to the whole class. This means that it has the same value for all the objects in the same class. They are also called static member variables. To clearly describe class variables, let's go back to our Car class example. Suppose that our Car class has one class variable called Count. If we change the value of Count to 2, all of the objects of the Car class will have the value 2 for their Count variable. Car Class Object Car A Object Car B Instance Variables Plate Number ABC 111 XYZ 123 Color Blue Red Manufacturer Mitsubishi Toyota Current Speed 50 km/h 100 km/h Class Variable Count = 2 Instance Methods Accelerate Method Turn Method Brake Method Table 19: Car class' methods and variables Introduction to Programming I 131
- 132. J.E.D.I 9.3.4 Class Instantiation To create an object or an instance of a class, we use the new operator. For example, if you want to create an instance of the class String, we write the following code, String str2 = new String(“Hello world!”); or also equivalent to, String str2 = "Hello"; The new operator allocates a memory for that object and returns a reference of that memory location to you. When you create an object, you actually invoke the class' constructor. The constructor is a method where you place all the initializations, it has the same name as the class. Introduction to Programming I 132 Figure 9.1: Classs Instantiation
- 133. J.E.D.I 9.4 Methods 9.4.1 What are Methods and Why Use Methods? In the examples we discussed before, we only have one method, and that is the main() method. In Java, we can define many methods which we can call from different methods. A method is a separate piece of code that can be called by a main program or any other method to perform some specific function. The following are characteristics of methods: • It can return one or no values • It may accept as many parameters it needs or no parameter at all. Parameters are also called function arguments. • After the method has finished execution, it goes back to the method that called it. Now, why do we need to create methods? Why don't we just place all the code inside one big method? The heart of effective problem solving is in problem decomposition. We can do this in Java by creating methods to solve a specific part of the problem. Taking a problem and breaking it into small, manageable pieces is critical to writing large programs. Introduction to Programming I 133
- 134. J.E.D.I 9.4.2 Calling Instance Methods and Passing Variables Now, to illustrate how to call methods, let's use the String class as an example. You can use the Java API documentation to see all the available methods in the String class. Later on, we will create our own methods, but for now, let us use what is available. To call an instance method, we write the following, nameOfObject.nameOfMethod( parameters ); Let's take two sample methods found in the class String, Method declaration Definition public char charAt(int index) Returns the character at the specified index. An index ranges from 0 to length() - 1. The first character of the sequence is at index 0, the next at index 1, and so on, as for array indexing. public boolean equalsIgnoreCase (String anotherString) Compares this String to another String, ignoring case considerations. Two strings are considered equal ignoring case if they are of the same length, and corresponding characters in the two strings are equal ignoring case. Table 20: Sample Methods of class String Using the methods, String str1 = "Hello"; char x = str2.charAt(0); //will return the character H //and store it to variable x String str2 = "hello"; //this will return a boolean value true boolean result = str1.equalsIgnoreCase( str1 ); Introduction to Programming I 134
- 135. J.E.D.I 9.4.3 Passing Variables in Methods In our examples, we already tried passing variables to methods. However, we haven't differentiated between the different types of variable passing in Java. There are two types of passing data to methods, the first one is pass-by-value and then, pass-by- reference. 9.4.3.1 Pass-by-value When a pass-by-value occurs, the method makes a copy of the value of the variable passed to the method. The method cannot accidentally modify the original argument even if it modifies the parameters during calculations. For example, public class TestPassByValue { public static void main( String[] args ){ int i = 10; //print the value of i System.out.println( i ); //call method test //and pass i to method test test( i ); //print the value of i. i not changed System.out.println( i ); } public static void test( int j ){ //change value of parameter j j = 33; } } In the given example, we called the method test and passed the value of i as parameter. The value of i is copied to the variable of the method j. Since j is the variable changed in the test method, it will not affect the variable value if i in main since it is a different copy of the variable. By default, all primitive data types when passed to a method are pass-by-value. Introduction to Programming I 135 Pass i as parameter which is copied to j
- 136. J.E.D.I 9.4.3.2 Pass-by-reference When a pass-by-reference occurs, the reference to an object is passed to the calling method. This means that, the method makes a copy of the reference of the variable passed to the method. However, unlike in pass-by-value, the method can modify the actual object that the reference is pointing to, since, although different references are used in the methods, the location of the data they are pointing to is the same. For example, class TestPassByReference { public static void main( String[] args ){ //create an array of integers int []ages = {10, 11, 12}; //print array values for( int i=0; i<ages.length; i++ ){ System.out.println( ages[i] ); } //call test and pass reference to array test( ages ); //print array values again for( int i=0; i<ages.length; i++ ){ System.out.println( ages[i] ); } } public static void test( int[] arr ){ //change values of array for( int i=0; i<arr.length; i++ ){ arr[i] = i + 50; } } } Introduction to Programming I 136 Pass ages as parameter which is copied to variable arr
- 137. J.E.D.I Coding Guidelines: A common misconception about pass-by-reference in Java is when creating a swap method using Java references. Take note that Java manipulates objects 'by reference,' but it passes object references to methods 'by value.'" As a result, you cannot write a standard swap method to swap objects. 9.4.4 Calling Static Methods Static methods are methods that can be invoked without instantiating a class (means without invoking the new keyword). Static methods belongs to the class as a whole and not to a certain instance (or object) of a class. Static methods are distinguished from instance methods in a class definition by the keyword static. To call a static method, just type, Classname.staticMethodName(params); Examples of static methods, we've used so far in our examples are, //prints data to screen System.out.println(“Hello world”); //converts the String 10, to an integer int i = Integer.parseInt(“10”); //Returns a String representation of the integer argument as an //unsigned integer base 16 String hexEquivalent = Integer.toHexString( 10 ); Introduction to Programming I 137 Figure 9.2: Pass-by-reference example
- 138. J.E.D.I 9.4.5 Scope of a variable In addition to a variable's data type and name, a variable has scope. The scope determines where in the program the variable is accessible. The scope also determines the lifetime of a variable or how long the variable can exist in memory. The scope is determined by where the variable declaration is placed in the program. To simplify things, just think of the scope as anything between the curly braces {...}. The outer curly braces is called the outer blocks, and the inner curly braces is called inner blocks. If you declare variables in the outer block, they are visible (i.e. usable) by the program lines inside the inner blocks. However, if you declare variables in the inner block, you cannot expect the outer block to see it. A variable's scope is inside the block where it is declared, starting from the point where it is declared, and in the inner blocks. For example, given the following code snippet, public class ScopeExample { public static void main( String[] args ){ int i = 0; int j = 0; //... some code here { int k = 0; int m = 0; int n = 0; } } The code we have here represents five scopes indicated by the lines and the letters representing the scope. Given the variables i,j,k,m and n, and the five scopes A,B,C,D and E, we have the following scopes for each variable: The scope of variable i is A. The scope of variable j is B. The scope of variable k is C. The scope of variable m is D. The scope of variable n is E. Introduction to Programming I 138 A E B D C
- 139. J.E.D.I Now, given the two methods main and test in our previous examples, class TestPassByReference { public static void main( String[] args ){ //create an array of integers int []ages = {10, 11, 12}; //print array values for( int i=0; i<ages.length; i++ ){ System.out.println( ages[i] ); } //call test and pass reference to array test( ages ); //print array values again for( int i=0; i<ages.length; i++ ){ System.out.println( ages[i] ); } } public static void test( int[] arr ){ //change values of array for( int i=0; i<arr.length; i++ ){ arr[i] = i + 50; } } } In the main method, the scope of the variables are, ages[] - scope A i in B - scope B i in C – scope C In the test method, the scope ofthe variables are, arr[] - scope D i in E - scope E Introduction to Programming I 139 A B C E D
- 140. J.E.D.I When declaring variables, only one variable with a given identifier or name can be declared in a scope. That means that if you have the following declaration, { int test = 10; int test = 20; } your compiler will generate an error since you should have unique names for your variables in one block. However, you can have two variables of the same name, if they are not declared in the same block. For example, int test = 0; System.out.print( test ); //..some code here { int test = 20; System.out.print( test ); } When the first System.out.print is invoke, it prints the value of the first test variable since it is the variable seen at that scope. For the second System.out.print, the value 20 is printed since it is the closest test variable seen at that scope. Coding Guidelines: Avoid having variables of the same name declared inside one method to avoid confusion. Introduction to Programming I 140
- 141. J.E.D.I 9.5 Casting, Converting and Comparing Objects In this section, we are going to learn how to do typecasting. Typecasting or casting is the process of converting a data of a certain data type to another data type. We will also learn how to convert primitive data types to objects and vice versa. And finally, we are going to learn how to compare objects. 9.5.1 Casting Primitive Types Casting between primitive types enables you to convert the value of one data from one type to another primitive type. This commonly occurs between numeric types. There is one primitive data type that we cannot do casting though, and that is the boolean data type. An example of typecasting is when you want to store an integer data to a variable of data type double. For example, int numInt = 10; double numDouble = numInt; //implicit cast In this example, since the destination variable (double) holds a larger value than what we will place inside it, the data is implicitly casted to data type double. Another example is when we want to typecast an int to a char value or vice versa. A character can be used as an int because each character has a corresponding numeric code that represents its position in the character set. If the variable i has the value 65, the cast (char)i produces the character value 'A'. The numeric code associated with a capital A is 65, according to the ASCII character set, and Java adopted this as part of its character support. For example, char valChar = 'A'; int valInt = valChar; System.out.print( valInt ); //explicit cast: output 65 Introduction to Programming I 141
- 142. J.E.D.I When we convert a data that has a large type to a smaller type, we must use an explicit cast. Explicit casts take the following form: (dataType)value where, dataType, is the name of the data type you're converting to value, is an expression that results in the value of the source type. For example, double valDouble = 10.12; int valInt = (int)valDouble; //convert valDouble to int type double x = 10.2; int y = 2; int result = (int)(x/y); //typecast result of operation to int Introduction to Programming I 142
- 143. J.E.D.I 9.5.2 Casting Objects Instances of classes also can be cast into instances of other classes, with one restriction: The source and destination classes must be related by inheritance; one class must be a subclass of the other. We'll cover more about inheritance later. Analogous to converting a primitive value to a larger type, some objects might not need to be cast explicitly. In particular, because a subclass contains all the same information as its superclass, you can use an instance of a subclass anywhere a superclass is expected. For example, consider a method that takes two arguments, one of type Object and another of type Window. You can pass an instance of any class for the Object argument because all Java classes are subclasses of Object. For the Window argument, you can pass in its subclasses, such as Dialog, FileDialog, and Frame. This is true anywhere in a program, not just inside method calls. If you had a variable defined as class Window, you could assign objects of that class or any of its subclasses to that variable without casting. This is true in the reverse, and you can use a superclass when a subclass is expected. There is a catch, however: Because subclasses contain more behavior than their superclasses, there's a loss in precision involved. Those superclass objects might not have all the behavior needed to act in place of a subclass object. For example, if you have an operation that calls methods in objects of the class Integer, using an object of class Number won't include many methods specified in Integer. Errors occur if you try to call methods that the destination object doesn't have. To use superclass objects where subclass objects are expected, you must cast them explicitly. You won't lose any information in the cast, but you gain all the methods and variables that the subclass defines. To cast an object to another class, you use the same operation as for primitive types: To cast, (classname)object where, classname, is the name of the destination class object, is a reference to the source object. Introduction to Programming I 143 Figure 9.3: Sample Class Hierarchy
- 144. J.E.D.I • Note: that casting creates a reference to the old object of the type classname; the old object continues to exist as it did before. The following example casts an instance of the class VicePresident to an instance of the class Employee; VicePresident is a subclass of Employee with more information, which here defines that the VicePresident has executive washroom privileges, Employee emp = new Employee(); VicePresident veep = new VicePresident(); emp = veep; // no cast needed for upward use veep = (VicePresident)emp; // must cast explicitlyCasting Introduction to Programming I 144 Figure 9.4: Class Hierarchy for superclass Employee
- 145. J.E.D.I 9.5.3 Converting Primitive Types to Objects and Vice Versa One thing you can't do under any circumstance is cast from an object to a primitive data type, or vice versa. Primitive types and objects are very different things in Java, and you can't automatically cast between the two or use them interchangeably. As an alternative, the java.lang package includes classes that correspond to each primitive data type: Float, Boolean, Byte, and so on. Most of these classes have the same names as the data types, except that the class names begin with a capital letter (Short instead of short, Double instead of double, and the like). Also, two classes have names that differ from the corresponding data type: Character is used for char variables and Integer for int variables. (Called Wrapper Classes) Java treats the data types and their class versions very differently, and a program won't compile successfully if you use one when the other is expected. Using the classes that correspond to each primitive type, you can create an object that holds the same value. Examples: //The following statement creates an instance of the Integer // class with the integer value 7801 (primitive -> Object) Integer dataCount = new Integer(7801); //The following statement converts an Integer object to // its primitive data type int. The result is an int with //value 7801 int newCount = dataCount.intValue(); // A common translation you need in programs // is converting a String to a numeric type, such as an int // Object->primitive String pennsylvania = "65000"; int penn = Integer.parseInt(pennsylvania); • CAUTION: The Void class represents nothing in Java, so there's no reason it would be used when translating between primitive values and objects. It's a placeholder for the void keyword, which is used in method definitions to indicate that the method does not return a value. Introduction to Programming I 145
- 146. J.E.D.I 9.5.4 Comparing Objects In our previous discussions, we learned about operators for comparing values—equal, not equal, less than, and so on. Most of these operators work only on primitive types, not on objects. If you try to use other values as operands, the Java compiler produces errors. The exceptions to this rule are the operators for equality: == (equal) and != (not equal). When applied to objects, these operators don't do what you might first expect. Instead of checking whether one object has the same value as the other object, they determine whether both sides of the operator refer to the same object. To compare instances of a class and have meaningful results, you must implement special methods in your class and call those methods. A good example of this is the String class. It is possible to have two different String objects that contain the same values. If you were to employ the == operator to compare these objects, however, they would be considered unequal. Although their contents match, they are not the same object. To see whether two String objects have matching values, a method of the class called equals() is used. The method tests each character in the string and returns true if the two strings have the same values. The following code illustrates this, class EqualsTest { public static void main(String[] arguments) { String str1, str2; str1 = "Free the bound periodicals."; str2 = str1; System.out.println("String1: " + str1); System.out.println("String2: " + str2); System.out.println("Same object? " + (str1 == str2)); str2 = new String(str1); System.out.println("String1: " + str1); System.out.println("String2: " + str2); System.out.println("Same object? " + (str1 == str2)); System.out.println("Same value? " + str1.equals(str2)); } } This program's output is as follows, OUTPUT: String1: Free the bound periodicals. String2: Free the bound periodicals. Same object? true String1: Free the bound periodicals. String2: Free the bound periodicals. Same object? false Same value? True Introduction to Programming I 146
- 147. J.E.D.I Now let's discuss the code. String str1, str2; str1 = "Free the bound periodicals."; The first part of this program declares two variables (str1 and str2), assigns the literal "Free the bound periodicals." to str1, and then assigns that value to str2. As you learned earlier, str1 and str2 now point to the same object, and the equality test proves that. str2 = new String(str1); In the second part of this program, you create a new String object with the same value as str1 and assign str2 to that new String object. Now you have two different string objects in str1 and str2, both with the same value. Testing them to see whether they're the same object by using the == operator returns the expected answer: false—they are not the same object in memory. Testing them using the equals() method also returns the expected answer: true—they have the same values. • NOTE: Why can't you just use another literal when you change str2, rather than using new? String literals are optimized in Java; if you create a string using a literal and then use another literal with the same characters, Java knows enough to give you the first String object back. Both strings are the same objects; you have to go out of your way to create two separate objects. Introduction to Programming I 147 Figure 9.5: Both references point to the same object Figure 9.6: References now point to different objects
- 148. J.E.D.I 9.5.5 Determining the Class of an Object Want to find out what an object's class is? Here's the way to do it for an object assigned to the variable key: 1. The getClass() method returns a Class object (where Class is itself a class) that has a method called getName(). In turn, getName() returns a string representing the name of the class. For Example, String name = key.getClass().getName(); 2. The instanceOf operator The instanceOf has two operands: a reference to an object on the left and a class name on the right. The expression returns true or false based on whether the object is an instance of the named class or any of that class's subclasses. For Example, boolean ex1 = "Texas" instanceof String; // true Object pt = new Point(10, 10); boolean ex2 = pt instanceof String; // false Introduction to Programming I 148
- 149. J.E.D.I 9.6 Exercises 9.6.1 Defining terms In your own words, define the following terms: 1. Class 2. Object 3. Instantiate 4. Instance Variable 5. Instance Method 6. Class Variables or static member variables 7. Constructor 9.6.2 Java Scavenger Hunt Pipoy is a newbie in the Java programming language. He just heard that there are already ready-to-use APIs in Java that one could use in their programs, and he's eager to try them out. The problem is, Pipoy does not have a copy of the Java Documentation, and he also doesn't have an internet access, so there's no way for him to view the Java APIs. Your task is to help Pipoy look for the APIs (Application Programming Interface). You should state the class where the method belongs, the method declaration and a sample usage of the said method. For example, if Pipoy wants to know the method that converts a String to integer, your answer should be: Class: Integer Method Declaration: public static int parseInt( String value ) Sample Usage: String strValue = "100"; int value = Integer.parseInt( strValue ); Make sure that the snippet of code you write in your sample usage compiles and outputs the correct answer, so as not to confuse Pipoy. (Hint: All methods are in the java.lang package). In cases where you can find more methods that can accomplish the task, give only one. Now let's start the search! 1. Look for a method that checks if a certain String ends with a certain suffix. For example, if the given string is "Hello", the method should return true the suffix given is "lo", and false if the given suffix is "alp". 2. Look for the method that determines the character representation for a specific digit in the specified radix. For example, if the input digit is 15, and the radix is 16, the method would return the character F, since F is the hexadecimal representation for the number 15 (base 10). 3. Look for the method that terminates the currently running Java Virtual Machine 4. Look for the method that gets the floor of a double value. For example, if I input a 3.13, the method should return the value 3. 5. Look for the method that determines if a certain character is a digit. For example, if I input '3', it returns the value true. Introduction to Programming I 149
- 150. J.E.D.I 10 Creating your own Classes 10.1 Objectives Now that we've studied on how to use existing classes from the Java class library, we will now be studying on how to write our own classes. For this section, in order to easily understand how to create classes, we will make a sample class wherein we will add more data and functionality as we go along the way. We will create a class that contains information of a Student and operations needed for a certain student record. Things to take note of for the syntax defined in this section and for the other sections: * - means that there may be 0 or more occurrences of the line whereit was applied to. <description> - indicates that you have to substitute an actual value for this part instead of typing it as it is. [] - indicates that this part is optional At the end of the lesson, the student should be able to: • Create their own classes • Declare attributes and methods for their classes • Use the this reference to access instance data • Create and call overloaded methods • Import and create packages • Use access modifiers to control access to class members Introduction to Programming I 150
- 151. J.E.D.I 10.2 Defining your own classes Before writing your class, think first on where you will be using your class and how your class will be used. Think of an appropriate name for the class, and list all the information or properties that you want your class to contain. Also list down the methods that you will be using for your class. To define a class, we write, <modifier> class <name> { <attributeDeclaration>* <constructorDeclaration>* <methodDeclaration>* } where <modifier> is an access modifier, which may be combined with other types of modifier. Coding Guidelines: Remember that for a top-level class, the only valid access modifiers are public and package (i.e., if no access modifier prefixes the class keyword). In this section, we will be creating a class that will contain a student record. Since we've already identified the purpose of our class, we can now name it. An appropriate name for our class would be StudentRecord. Now, to define our class we write, public class StudentRecord { //we'll add more code here later } where, public - means that our class is accessible to other classes outside the package class - this is the keyword used to create a class in Java StudentRecord - a unique identifier that describes our class Coding Guidelines: 1. Think of an appropriate name for your class. Don't just call your class XYZ or any random names you can think of. 2. Class names should start with a CAPITAL letter. 3. The filename of your class should have the SAME NAME as your public class name. Introduction to Programming I 151
- 152. J.E.D.I 10.3 Declaring Attributes To declare a certain attribute for our class, we write, <modifier> <type> <name> [= <default_value>]; Now, let us write down the list of attributes that a student record can contain. For each information, also list what data types would be appropriate to use. For example, you don't want to have a data type int for a student's name, or a String for a student's grade. The following are some sample information we want to add to the student record. name - String address - String age - int math grade - double english grade - double science grade - double average grade - double You can add more information if you want to, it's all really up to you. But for this example, we will be using these information. 10.3.1 Instance Variables Now that we have a list of all the attributes we want to add to our class, let us now add them to our code. Since we want these attributes to be unique for each object (or for each student), we should declare them as instance variables. For example, public class StudentRecord { private String name; private String address; private int age; private double mathGrade; private double englishGrade; private double scienceGrade; private double average; //we'll add more code here later } where, private here means that the variables are only accessible within the class. Other objects cannot access these variables directly. We will cover more about accessibility later. Coding Guidelines: 1. Declare all your instance variables on the top of the class declaration. 2. Declare one variable for each line. 3. Instance variables, like any other variables should start with a SMALL letter. 4. Use an appropriate data type for each variable you declare. 5. Declare instance variables as private so that only class methods can access them directly. Introduction to Programming I 152
- 153. J.E.D.I 10.3.2 Class Variables or Static Variables Aside from instance variables, we can also declare class variables or variables that belong to the class as a whole. The value of these variables are the same for all the objects of the same class. Now suppose, we want to know the total number of student records we have for the whole class, we can declare one static variable that will hold this value. Let us call this as studentCount. To declare a static variable, public class StudentRecord { //instance variables we have declared private static int studentCount; //we'll add more code here later } we use the keyword static to indicate that a variable is a static variable. So far, our whole code now looks like this. public class StudentRecord { private String name; private String address; private int age; private double mathGrade; private double englishGrade; private double scienceGrade; private double average; private static int studentCount; //we'll add more code here later } 10.4 Declaring Methods Before we discuss what methods we want our class to have, let us first take a look at the general syntax for declaring methods. To declare methods we write, <modifier> <returnType> <name>(<parameter>*) { <statement>* } where, <modifier> can carry a number of different modifiers <returnType> can be any data type (including void) <name> can be any valid identifier <parameter> ::= <parameter_type> <parameter_name>[,] Introduction to Programming I 153
- 154. J.E.D.I 10.4.1 Accessor methods In order to implement encapsulation, that is, we don't want any objects to just access our data anytime, we declare the fields or attributes of our classes as private. However, there are times wherein we want other objects to access private data. In order to do this, we create accessor methods. Accessor methods are used to read values from class variables (instance/static). An accessor method usually starts with a get<NameOfInstanceVariable>. It also returns a value. For our example, we want an accessor method that can read the name, address, english grade, math grade and science grade of the student. Now let's take a look at one implementation of an accessor method, public class StudentRecord { private String name; : : public String getName(){ return name; } } where, public - means that the method can be called from objects outside the class String - is the return type of the method. This means that the method should return a value of type String getName - the name of the method () - this means that our method does not have any parameters The statement, return name; in our program signifies that it will return the value of the instance variable name to the calling method. Take note that the return type of the method should have the same data type as the data in the return statement. You usually encounter the following error if the two does not have the same data type, StudentRecord.java:14: incompatible types found : int required: java.lang.String return age; ^ 1 error Introduction to Programming I 154
- 155. J.E.D.I Another example of an accessor method is the getAverage method, public class StudentRecord { private String name; : : public double getAverage(){ double result = 0; result = ( mathGrade+englishGrade+scienceGrade )/3; return result; } } The getAverage method computes the average of the 3 grades and returns the result. 10.4.2 Mutator Methods Now, what if we want other objects to alter our data? What we do is we provide methods that can write or change values of our class variables (instance/static). We call these methods, mutator methods. A mutator method is usuallyu written as set<NameOfInstanceVariable>. Now let's take a look at one implementation of a mutator method, public class StudentRecord { private String name; : : public void setName( String temp ){ name = temp; } } where, public - means that the method can be called from objects outside the class void - imeans that the method does not return any value setName - the name of the method (String temp) - parameter that will be used inside our method The statement, name = temp; assigns the value of temp to name and thus changes the data inside the instance variable name. Take note that mutator methods don't return values. However, it contains some program argument or arguments that will be used inside the method. Introduction to Programming I 155
- 156. J.E.D.I 10.4.3 Multiple Return statements You can have multiple return statements for a method as long as they are not on the same block. You can also use constants to return values instead of variables. For example, consider the method, public String getNumberInWords( int num ){ String defaultNum = "zero"; if( num == 1 ){ return "one"; //return a constant } else if( num == 2){ return "two"; //return a constant } //return a variable return defaultNum; } 10.4.4 Static methods For the static variable studentCount, we can create a static method to access its value. public class StudentRecord { private static int studentCount; public static int getStudentCount(){ return studentCount; } } where, public - means that the method can be called from objects outside the class static - means that the method is static and should be called by typing,[ClassName].[methodName]. For example, in this case, we call the method StudentRecord.getStudentCount() int - is the return type of the method. This means that the method should return a value of type int getStudentCount - the name of the method () - this means that our method does not have any parameters For now, getStudentCount will always return the value zero since we haven't done anything yet in our program in order to set its value. We will try to change the value of studentCount later on when we discuss constructors. Coding Guidelines: 1. Method names should start with a SMALL letter. 2. Method names should be verbs 3. Always provide documentation before the declaration of the method. You can use javadocs style for this. Please see example. Introduction to Programming I 156
- 157. J.E.D.I 10.4.5 Sample Source Code for StudentRecord class Here is the code for our StudentRecord class, public class StudentRecord { private String name; private String address; private int age; private double mathGrade; private double englishGrade; private double scienceGrade; private double average; private static int studentCount; /** * Returns the name of the student */ public String getName(){ return name; } /** * Changes the name of the student */ public void setName( String temp ){ name = temp; } // other code here .... /** * Computes the average of the english, math and science * grades */ public double getAverage(){ double result = 0; result = ( mathGrade+englishGrade+scienceGrade )/3; return result; } /** * returns the number of instances of StudentRecords */ public static int getStudentCount(){ return studentCount; } } Introduction to Programming I 157
- 158. J.E.D.I Now, here's a sample code of a class that uses our StudentRecord class. public class StudentRecordExample { public static void main( String[] args ){ //create three objects for Student record StudentRecord annaRecord = new StudentRecord(); StudentRecord beahRecord = new StudentRecord(); StudentRecord crisRecord = new StudentRecord(); //set the name of the students annaRecord.setName("Anna"); beahRecord.setName("Beah"); crisRecord.setName("Cris"); //print anna's name System.out.println( annaRecord.getName() ); //print number of students System.out.println("Count="+StudentRecord.getStudentCount()); } } The output of this program is, Anna Student Count = 0 Introduction to Programming I 158
- 159. J.E.D.I 10.5 The this reference The this reference is used to access the instance variables shadowed by the parameters. To understand this better, let's take for example the setAge method. Suppose we have the following declaration for setAge. public void setAge( int age ){ age = age; //WRONG!!! } The parameter name in this declaration is age, which has the same name as the instance variable age. Since the parameter age is the closest declaration to the method, the value of the parameter age will be used. So in the statement, age = age; we are just assigning the value of the parameter age to itself! This is not what we want to happen in our code. In order to correct this mistake, we use the this reference. To use the this reference, we type, this.<nameOfTheInstanceVariable> So for example, we can now rewrite our code to, public void setAge( int age ){ this.age = age; } This method will then assign the value of the parameter age to the instance variable of the object StudentRecord. NOTE: You can only use the this reference for instance variables and NOT static or class variables. Introduction to Programming I 159
- 160. J.E.D.I 10.6 Overloading Methods In our classes, we want to sometimes create methods that has the same names but function differently depending on the parameters that are passed to them. This capability is possible in Java, and it is called Method Overloading. Method overloading allows a method with the same name but different parameters, to have different implementations and return values of different types. Rather than invent new names all the time, method overloading can be used when the same operation has different implementations. For example, in our StudentRecord class we want to have a method that prints information about the student. However, we want the print method to print things differently depending on the parameters we pass to it. For example, when we pass a String, we want the print method to print out the name, address and age of the student. When we pass 3 double values, we want the method to print the student's name and grades. We have the following overloaded methods inside our StudentRecord class, public void print( String temp ){ System.out.println("Name:" + name); System.out.println("Address:" + address); System.out.println("Age:" + age); } public void print(double eGrade, double mGrade, double sGrade) System.out.println("Name:" + name); System.out.println("Math Grade:" + mGrade); System.out.println("English Grade:" + eGrade); System.out.println("Science Grade:" + sGrade); } Introduction to Programming I 160
- 161. J.E.D.I When we try to call this in the following main method, public static void main( String[] args ) { StudentRecord annaRecord = new StudentRecord(); annaRecord.setName("Anna"); annaRecord.setAddress("Philippines"); annaRecord.setAge(15); annaRecord.setMathGrade(80); annaRecord.setEnglishGrade(95.5); annaRecord.setScienceGrade(100); //overloaded methods annaRecord.print( annaRecord.getName() ); annaRecord.print( annaRecord.getEnglishGrade(), annaRecord.getMathGrade(), annaRecord.getScienceGrade()); } we will have the output for the first call to print, Name:Anna Address:Philippines Age:15 we will have the output for the second call to print, Name:Anna Math Grade:80.0 English Grade:95.5 Science Grade:100.0 Always remember that overloaded methods have the following properties, – the same name – different parameters – return types can be different or the same Introduction to Programming I 161
- 162. J.E.D.I 10.7 Declaring Constructors We have discussed before the concept of constructors. Constructors are important in instantiating an object. It is a method where all the initializations are placed. The following are the properties of a constructor: 1. Constructors have the same name as the class 2. A constructor is just like an ordinary method, however only the following information can be placed in the header of the constructor, scope or accessibility identifier (like public...), constructor's name and parameters if it has any. 3. Constructors does not have any return value 4. You cannot call a constructor directly, it can only be called by using the new operator during class instantiation. To declare a constructor, we write, <modifier> <className> (<parameter>*) { <statement>* } 10.7.1 Default Constructor Every class has a default constructor. The default constructor is the constructor without any parameters. If the class does not specify any constructors, then an implicit default constructor is created. For example, in our StudentRecord class, the default constructor would look like this, public StudentRecord() { //some code here } 10.7.2 Overloading Constructors As we have mentioned, constructors can also be overloaded, for example, we have here four overloaded constructors, public StudentRecord(){ //some initialization code here } public StudentRecord(String temp){ this.name = temp; } public StudentRecord(String name, String address){ this.name = name; this.address = address; } public StudentRecord(double mGrade, double eGrade, double sGrade){ mathGrade = mGrade; englishGrade = eGrade; scienceGrade = sGrade; } Introduction to Programming I 162
- 163. J.E.D.I 10.7.3 Using Constructors To use these constructors, we have the following code, public static void main( String[] args ) { //create three objects for Student record StudentRecord annaRecord=new StudentRecord("Anna"); StudentRecord beahRecord=new StudentRecord("Beah", "Philippines"); StudentRecord crisRecord=new StudentRecord(80,90,100); //some code here } Now, before we move on, let us go back to the static variable studentCount we have declared a while ago. The purpose of the studentCount is to count the number of objects that are instantiated with the class StudentRecord. So, what we want to do here is, everytime an object of class StudentRecord is instantiated, we increment the value of studentCount. A good location to modify and increment the value of studentCount is in the constructors, because it is always called everytime an object is instantiated. For example, public StudentRecord(){ //some initialization code here studentCount++; //add a student } public StudentRecord(String temp){ this.name = temp; studentCount++; //add a student } public StudentRecord(String name, String address){ this.name = name; this.address = address; studentCount++; //add a student } public StudentRecord(double mGrade, double eGrade, double sGrade){ mathGrade = mGrade; englishGrade = eGrade; scienceGrade = sGrade; studentCount++; //add a student } Introduction to Programming I 163
- 164. J.E.D.I 10.7.4 The this() Constructor Call Constructor calls can be chained, meaning, you can call another constructor from inside another constructor. We use the this() call for this. For example, given the following code, 1: public StudentRecord(){ 2: this("some string"); 3: 4: } 5: 6: public StudentRecord(String temp){ 7: this.name = temp; 8: } 9: 10: public static void main( String[] args ) 11: { 12: 13: StudentRecord annaRecord = new StudentRecord(); 14: } Given the code above, when the statement at line 13 is called, it will call the default constructor line 1. When statement in line 2 is executed, it will then call the constructor that has a String parameter (in line 6). There are a few things to remember when using the this constructor call: 1. When using the this constructor call, IT MUST OCCUR AS THE FIRST STATEMENT in a constructor 2. It can ONLY BE USED IN A CONSTRUCTOR DEFINITION. The this call can then be followed by any other relevant statements. Introduction to Programming I 164
- 165. J.E.D.I 10.8 Packages Packages are Java’s means of grouping related classes and interfaces together in a single unit (interfaces will be discussed later). This powerful feature provides for a convenient mechanism for managing a large group of classes and interfaces while avoiding potential naming conflicts. 10.8.1 Importing Packages To be able to use classes outside of the package you are currently working in, you need to import the package of those classes. By default, all your Java programs import the java.lang.* package, that is why you can use classes like String and Integers inside the program eventhough you haven't imported any packages. The syntax for importing packages is as follows, import <nameOfPackage>; For example, if you want to use the class Color inside package awt, you have to type the following, import java.awt.Color; import java.awt.*; The first statement imports the specific class Color while the other imports all of the classes in the java.awt package. Another way to import classes from other packages is through explicit package referencing. This is done by using the package name to declare an object of a class. java.awt.Color color; 10.8.2 Creating your own packages To create our own package, we write, package <packageName>; Suppose we want to create a package where we will place our StudentRecord class, together with other related classes. We will call our package, schoolClasses. The first thing you have to do is create a folder named schoolClasses. Copy all the classes that you want to belong to this package inside this folder. After copying, add the following code at the top of the class file. For example, package schoolClasses; public class StudentRecord { private String name; private String address; private int age; : Packages can also be nested. In this case, the Java interpreter expects the directory structure containing the executable classes to match the package hierarchy. Introduction to Programming I 165
- 166. J.E.D.I 10.8.3 Setting the CLASSPATH Now, suppose we place the package schoolClasses under the C: directory. We need to set the classpath to point to that directory so that when we try to run it, the JVM will be able to see where our classes are stored. Before we discuss how to set the classpath, let us take a look at an example on what will happen if we don't set the classpath. Suppose we compile and then run the StudentRecord class we wrote in the last section, C:schoolClasses>javac StudentRecord.java C:schoolClasses>java StudentRecord Exception in thread "main" java.lang.NoClassDefFoundError: StudentRecord (wrong name: schoolClasses/StudentRecord) at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(Unknown Source) at java.security.SecureClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.access$100(Unknown Source) at java.net.URLClassLoader$1.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClass(Unknown Source) at java.lang.ClassLoader.loadClassInternal(Unknown Source) We encounter a NoClassDefFoundError which means that Java did not know where to look for your class. The reason for this is that your class StudentRecord now belongs to a package named studentClasses. If we want to run our class, we jave to tell Java about its full class name which is schoolClasses.StudentRecord. We also have to tell JVM where to look for our packages, which in this case is in location C:. To do this, we must set the classpath. To set the classpath in Windows, we type this at the command prompt, C:schoolClasses> set classpath=C: where C: is the directory in which we have placed the packages. After setting the classpath, we can now run our program anywhere by typing, C:schoolClasses> java schoolClasses.StudentRecord For Unix base systems, suppose we have our classes in the directory /usr/local/myClasses, we write, export classpath=/usr/local/myClasses Introduction to Programming I 166
- 167. J.E.D.I Take note that you can set the classpath anywhere. You can also set more than one classpath, we just have to separate them by ;(for windows) and : (for Unix based systems). For example, set classpath=C:myClasses;D:;E:MyProgramsJava and for Unix based systems, export classpath=/usr/local/java:/usr/myClasses Introduction to Programming I 167
- 168. J.E.D.I 10.9 Access Modifiers When creating our classes and defining the properties and methods in our class, we want to implement some kind of restriction to access these data. For example, if you want a certain attribute to be changed only by the methods inside the class, you may want to hide this from other objects using your class. In Java, we have what we call access modifiers in order to implement this. There are four different types of member access modifiers in Java: public, private, protected and default. The first three access modifiers are explicitly written in the code to indicate the access type, for the fourth one which is default, no keyword is used. 10.9.1 default access (also called package accessibility) This specifies that only classes in the same package can have access to the class' variables and methods. There are no actual keyword for the default modifier; it is applied in the absence of an access modifier. For example, public class StudentRecord { //default access to instance variable int name; //default access to method String getName(){ return name; } } In this example, the instance variable name and the method getName() can be accessed from other objects, as long as the object belongs to the same package where the class StudentRecord belongs to. 10.9.2 public access This specifies that class members are accessible to anyone, both inside and outside the class. Any object that interacts with the class can have access to the public members of the class. For example, public class StudentRecord { //default access to instance variable public int name; //default access to method public String getName(){ return name; } } In this example, the instance variable name and the method getName() can be accessed from other objects. Introduction to Programming I 168
- 169. J.E.D.I 10.9.3 protected access This specifies that the class members are accessible only to methods in that class and the subclasses of the class. For example, public class StudentRecord { //default access to instance variable protected int name; //default access to method protected String getName(){ return name; } } In this example, the instance variable name and the method getName() can be accessed only from methods inside the class and from subclasses of StudentRecord. We will discuss about subclasses on the next chapter. 10.9.4 private access This specifies that the class members are only accessible by the class they are defined in. For example, public class StudentRecord { //default access to instance variable private int name; //default access to method private String getName(){ return name; } } In this example, the instance variable name and the method getName() can be accessed only from methods inside the class. Coding Guidelines: The instance variables of a class should normally be declared private, and the class will just provide accessor and mutator methods to these variables. Introduction to Programming I 169
- 170. J.E.D.I 10.10 Exercises 10.10.1 Address Book Entry Your task is to create a class that contains an address book entry. The following table describes the information that an adressbook entry has. Attributes/Properties Description Name Name of the person in the addressbook Address Address of the person Telephone Number Telephone number of the person Email Address Person's Email address Table 21: Attributes and Attributes Descriptions For the methods, create the following: 1. Provide the necessary accessor and mutator methods for all the attributes. 2. Constructors 10.10.2 AddressBook Create a class address book that can contain 100 entries of AddressBookEntry objects (use the class you created in the first exercise). You should provide the following methods for the address book. 1. Add entry 2. Delete entry 3. View all entries 4. Update an entry Introduction to Programming I 170
- 171. J.E.D.I 11 Inheritance, Polymorphism and Interfaces 11.1 Objectives In this section, we will be discussing on how a class can inherit the properties of an existing class. A class that does this is called a subclass and its parent class is called the superclass. We will also be discussing a special property of Java wherein it can automatically apply the proper methods to each object regardless of what subclass the object came from. This property is known as polymorphism. Finally, we are going to discusss about interfaces that helps reduce programming effort. At the end of the lesson, the student should be able to: • Define super classes and subclasses • Override methods of superclasses • Create final methods and final classes 11.2 Inheritance In Java, all classes, including the classes that make up the Java API, are subclassed from the Object superclass. A sample class hierarchy is shown below. Any class above a specific class in the class hierarchy is known as a superclass. While any class below a specific class in the class hierarchy is known as a subclass of that class. Inheritance is a major advantage in object-oriented programming since once a behavior (method) is defined in a superclass, that behavior is automatically inherited by all subclasses. Thus, you can encode a method only once and they can be used by all subclasses. A subclass only need to implement the differences between itself and the parent. Introduction to Programming I 171 Figure 11.1: Class Hierarchy
- 172. J.E.D.I 11.2.1 Defining Superclasses and Subclasses To derive a class, we use the extends keyword. In order to illustrate this, let's create a sample parent class. Suppose we have a parent class called Person. public class Person { protected String name; protected String address; /** * Default constructor */ public Person(){ System.out.println(“Inside Person:Constructor”); name = ""; address = ""; } /** * Constructor with 2 parameters */ public Person( String name, String address ){ this.name = name; this.address = address; } /** * Accessor methods */ public String getName(){ return name; } public String getAddress(){ return address; } public void setName( String name ){ this.name = name; } public void setAddress( String add ){ this.address = add; } } Notice that, the attributes name and address are declared as protected. The reason we did this is that, we want these attributes to be accessible by the subclasses of the superclass. If we declare this as private, the subclasses won't be able to use them. Take note that all the properties of a superclass that are declared as public, protected and default can be accessed by its subclasses. Introduction to Programming I 172
- 173. J.E.D.I Now, we want to create another class named Student. Since a student is also a person, we decide to just extend the class Person, so that we can inherit all the properties and methods of the existing class Person. To do this, we write, public class Student extends Person { public Student(){ System.out.println(“Inside Student:Constructor”); //some code here } // some code here } When a Student object is instantiated, the default constructor of its superclass is invoked implicitly to do the necessary initializations. After that, the statements inside the subclass are executed. To illustrate this, consider the following code, public static void main( String[] args ){ Student anna = new Student(); } In the code, we create an object of class Student. The output of the program is, Inside Person:Constructor Inside Student:Constructor The program flow is shown below. Introduction to Programming I 173 Figure 11.2: Program Flow
- 174. J.E.D.I 11.2.2 The super keyword A subclass can also explicitly call a constructor of its immediate superclass. This is done by using the super constructor call. A super constructor call in the constructor of a subclass will result in the execution of relevant constructor from the superclass, based on the arguments passed. For example, given our previous example classes Person and Student, we show an example of a super constructor call. Given the following code for Student, public Student(){ super( "SomeName", "SomeAddress" ); System.out.println("Inside Student:Constructor"); } This code calls the second constructor of its immediate superclass (which is Person) and executes it. Another sample code shown below, public Student(){ super(); System.out.println("Inside Student:Constructor"); } This code calls the default constructor of its immediate superclass (which is Person) and executes it. There are a few things to remember when using the super constructor call: 1. The super() call MUST OCCUR THE FIRST STATEMENT IN A CONSTRUCTOR. 2. The super() call can only be used in a constructor definition. 3. This implies that the this() construct and the super() calls CANNOT BOTH OCCUR IN THE SAME CONSTRUCTOR. Another use of super is to refer to members of the superclass (just like the this reference ). For example, public Student() { super.name = “somename”; super.address = “some address”; } Introduction to Programming I 174
- 175. J.E.D.I 11.2.3 Overriding Methods If for some reason a derived class needs to have a different implementation of a certain method from that of the superclass, overriding methods could prove to be very useful. A subclass can override a method defined in its superclass by providing a new implementation for that method. Suppose we have the following implementation for the getName method in the Person superclass, public class Person { : : public String getName(){ System.out.println("Parent: getName"); return name; } : } To override, the getName method in the subclass Student, we write, public class Student extends Person { : : public String getName(){ System.out.println("Student: getName"); return name; } : } So, when we invoke the getName method of an object of class Student, the overridden method would be called, and the output would be, Student: getName Introduction to Programming I 175
- 176. J.E.D.I 11.2.4 Final Methods and Final Classes In Java, it is also possible to declare classes that can no longer be subclassed. These classes are called final classes. To declare a class to be final, we just add the final keyword in the class declaration. For example, if we want the class Person to be declared final, we write, public final class Person { //some code here } Many of the classes in the Java API are declared final to ensure that their behavior cannot be overridden. Examples of these classes are Integer, Double and String. It is also possible in Java to create methods that cannot be overridden. These methods are what we call final methods. To declare a method to be final, we just add the final keyword in the method declaration. For example, if we want the getName method in class Person to be declared final, we write, public final String getName(){ return name; } Static methods are automatically final. This means that you cannot override them. Introduction to Programming I 176
- 177. J.E.D.I 11.3 Polymorphism Now, given the parent class Person and the subclass Student of our previous example, we add another subclass of Person which is Employee. Below is the class hierarchy for that, In Java, we can create a reference that is of type superclass to an object of its subclass. For example, public static main( String[] args ) { Person ref; Student studentObject = new Student(); Employee employeeObject = new Employee(); ref = studentObject; //Person ref points to a // Student object //some code here } Now suppose we have a getName method in our superclass Person, and we override this method in both the subclasses Student and Employee, public class Person { public String getName(){ System.out.println(“Person Name:” + name); return name; } } public class Student extends Person { public String getName(){ System.out.println(“Student Name:” + name); return name; } } public class Employee extends Person { public String getName(){ System.out.println(“Employee Name:” + name); return name; } } Introduction to Programming I 177 Figure 11.3: Hierarchy for Person class and it's classes Person Student Employee
- 178. J.E.D.I Going back to our main method, when we try to call the getName method of the reference Person ref, the getName method of the Student object will be called. Now, if we assign ref to an Employee object, the getName method of Employee will be called. public static main( String[] args ) { Person ref; Student studentObject = new Student(); Employee employeeObject = new Employee(); ref = studentObject; //Person reference points to a // Student object String temp = ref.getName(); //getName of Student //class is called System.out.println( temp ); ref = employeeObject; //Person reference points to an // Employee object String temp = ref.getName(); //getName of Employee //class is called System.out.println( temp ); } This ability of our reference to change behavior according to what object it is holding is called polymorphism. Polymorphism allows multiple objects of different subclasses to be treated as objects of a single superclass, while automatically selecting the proper methods to apply to a particular object based on the subclass it belongs to. Another example that exhibits the property of polymorphism is when we try to pass a reference to methods. Suppose we have a static method printInformation that takes in a Person object as reference, we can actually pass a reference of type Employee and type Student to this method as long as it is a subclass of the class Person. public static main( String[] args ) { Student studentObject = new Student(); Employee employeeObject = new Employee(); printInformation( studentObject ); printInformation( employeeObject ); } public static printInformation( Person p ){ . . . . } Introduction to Programming I 178
- 179. J.E.D.I 11.4 Abstract Classes Now suppose we want to create a superclass wherein it has certain methods in it that contains some implementation, and some methods wherein we just want to be overridden by its subclasses. For example, we want to create a superclass named LivingThing. This class has certain methods like breath, eat, sleep and walk. However, there are some methods in this superclass wherein we cannot generalize the behavior. Take for example, the walk method. Not all living things walk the same way. Take the humans for instance, we humans walk on two legs, while other living things like dogs walk on four legs. However, there are many characteristics that living things have in common, that is why we want to create a general superclass for this. In order to do this, we can create a superclass that has some methods with implementations and others which do not. This kind of class is called an abstract class. An abstract class is a class that cannot be instantiated. It often appears at the top of an object-oriented programming class hierarchy, defining the broad types of actions possible with objects of all subclasses of the class. Those methods in the abstract classes that do not have implementation are called abstract methods. To create an abstract method, just write the method declaration without the body and use the abstract keyword. For example, public abstract void someMethod(); Introduction to Programming I 179 Figure 11.4: Abstract class
- 180. J.E.D.I Now, let's create an example abstract class. public abstract class LivingThing { public void breath(){ System.out.println("Living Thing breathing..."); } public void eat(){ System.out.println("Living Thing eating..."); } /** * abstract method walk * We want this method to be overridden by subclasses of * LivingThing */ public abstract void walk(); } When a class extends the LivingThing abstract class, it is required to override the abstract method walk(), or else, that subclass will also become an abstract class, and therefore cannot be instantiated. For example, public class Human extends LivingThing { public void walk(){ System.out.println("Human walks..."); } } If the class Human does not override the walk method, we would encounter the following error message, Human.java:1: Human is not abstract and does not override abstract method walk() in LivingThing public class Human extends LivingThing ^ 1 error Coding Guidelines: Use abstract classes to define broad types of behaviors at the top of an object-oriented programming class hierarchy, and use its subclasses to provide implementation details of the abstract class. Introduction to Programming I 180
- 181. J.E.D.I 11.5 Interfaces An interface is a special kind of block containing method signatures (and possibly constants) only. Interfaces define the signatures of a set of methods without the body. Interfaces define a standard and public way of specifying the behavior of classes. They allow classes, regardless of their location in the class hierarchy, to implement common behaviors. Note that interfaces exhibit polymorphism as well, since program may call an interface method and the proper version of that method will be executed depending on the type of object passed to the interface method call. 11.5.1 Why do we use Interfaces? We need to use interfaces if we want unrelated classes to implement similar methods. Thru interfaces, we can actually capture similarities among unrelated classes without artificially forcing a class relationship. Let's take as an example a class Line which contains methods that computes the length of the line and compares a Line object to objects of the same class. Now, suppose we have another class MyInteger which contains methods that compares a MyInteger object to objects of the same class. As we can see here, both of the classes have some similar methods which compares them from other objects of the same type, but they are not related whatsoever. In order to enforce a way to make sure that these two classes implement some methods with similar signatures, we can use an interface for this. We can create an interface class, let's say interface Relation which has some comparison method declarations. Our interface Relation can be declared as, public interface Relation { public boolean isGreater( Object a, Object b); public boolean isLess( Object a, Object b); public boolean isEqual( Object a, Object b); } Another reason for using an object's programming interface is to reveal an object's programming interface without revealing its class. As we can see later on the section Interface vs. Classes, we can actually use an interface as data type. Finally, we need to use interfaces to model multiple inheritance which allows a class to have more than one superclass. Multiple inheritance is not present in Java, but present in other object-oriented languages like C++. 11.5.2 Interface vs. Abstract Class The following are the main differences between an interface and an abstract class: interface methods have no body, an interface can only define constants and an interface have no direct inherited relationship with any particular class, they are defined independently. Introduction to Programming I 181
- 182. J.E.D.I 11.5.3 Interface vs. Class One common characteristic of an interface and class is that they are both types. This means that an interface can be used in places where a class can be used. For example, given a class Person and an interface PersonInterface, the following declarations are valid: PersonInterface pi = new Person(); Person pc = new Person(); However, you cannot create an instance from an interface. An example of this is: PersonInterface pi = new PersonInterface(); //COMPILE //ERROR!!! Another common characteristic is that both interface and class can define methods. However, an interface does not have an implementation code while the class have one. 11.5.4 Creating Interfaces To create an interface, we write, public interface [InterfaceName] { //some methods without the body } As an example, let's create an interface that defines relationships between two objects according to the “natural order” of the objects. public interface Relation { public boolean isGreater( Object a, Object b); public boolean isLess( Object a, Object b); public boolean isEqual( Object a, Object b); } Now, to use the interface, we use the implements keyword. For example, /** * This class defines a line segment */ public class Line implements Relation { private double x1; private double x2; private double y1; private double y2; public Line(double x1, double x2, double y1, double y2){ this.x1 = x1; this.x2 = x2; this.y1 = y1; this.y2 = y2; } public double getLength(){ Introduction to Programming I 182
- 183. J.E.D.I double length = Math.sqrt((x2-x1)*(x2-x1) + (y2-y1)* (y2-y1)); return length; } public boolean isGreater( Object a, Object b){ double aLen = ((Line)a).getLength(); double bLen = ((Line)b).getLength(); return (aLen > bLen); } public boolean isLess( Object a, Object b){ double aLen = ((Line)a).getLength(); double bLen = ((Line)b).getLength(); return (aLen < bLen); } public boolean isEqual( Object a, Object b){ double aLen = ((Line)a).getLength(); double bLen = ((Line)b).getLength(); return (aLen == bLen); } } When your class tries to implement an interface, always make sure that you implement all the methods of that interface, or else, you would encounter this error, Line.java:4: Line is not abstract and does not override abstract method isGreater(java.lang.Object,java.lang.Object) in Relation public class Line implements Relation ^ 1 error Coding Guidelines: Use interfaces to create the same standard method definitions in may different classes. Once a set of standard method definition is created, you can write a single method to manipulate all of the classes that implement the interface. Introduction to Programming I 183
- 184. J.E.D.I 11.5.5 Relationship of an Interface to a Class As we have seen in the previous section, a class can implement an interface as long as it provides the implementation code for all the methods defined in the interface. Another thing to note about the relationship of interfaces to classes is that, a class can only EXTEND ONE super class, but it can IMPLEMENT MANY interfaces. An example of a class that implements many interfaces is, public class Person implements PersonInterface, LivingThing, WhateverInterface { //some code here } Another example of a class that extends one super class and implements an interface is, public class ComputerScienceStudent extends Student implements PersonInterface, LivingThing { //some code here } Take note that an interface is not part of the class inheritance hierarchy. Unrelated classes can implement the same interface. 11.5.6 Inheritance among Interfaces Interfaces are not part of the class hierarchy. However, interfaces can have inheritance relationship among themselves. For example, suppose we have two interfaces StudentInterface and PersonInterface. If StudentInterface extends PersonInterface, it will inherit all of the method declarations in PersonInterface. public interface PersonInterface { . . . } public interface StudentInterface extends PersonInterface { . . . } Introduction to Programming I 184
- 185. J.E.D.I 11.6 Exercises 11.6.1 Extending StudentRecord In this exercise, we want to create a more specialized student record that contains additional information about a Computer Science student. Your task is to extend the StudentRecord class that was implemented in the previous lessons. Add some attributes and methods that you think are needed for a Computer Science student record. Try to override some existing methods in the superclass StudentRecord, if you really need to. 11.6.2 The Shape abstract class Try to create an abstract class called Shape with abstract methods getArea() and getName(). Write two of its subclasses Circle and Square. You can add additional methods to its subclasses if you want to. Introduction to Programming I 185
- 186. J.E.D.I 12 Basic Exception Handling 12.1 Objectives In this section, we are going to study a technique used in Java to handle unusual conditions that interrupt the normal operation of the program. This technique is called exception handling. At the end of the lesson, the student should be able to: • Define exceptions • Handle exceptions using a simple try-catch-finally block 12.2 What are Exceptions? An exception is an event that interrupts the normal processing flow of a program. This event is usually some error of some sort. This causes our program to terminate abnormally. Some examples of exceptions that you might have encountered in our previous exercises are: ArrayIndexOutOfBounds exceptions, which occurs if we try to access a non-existent array element, or maybe a NumberFormatException, which occurs when we try to pass as a parameter a non-number in the Integer.parseInt method. 12.3 Handling Exceptions To handle exceptions in Java, we use a try-catch-finally block. What we do in our programs is that we place the statements that can possibly generate an exception inside this block. The general form of a try-catch-finally block is, try{ //write the statements that can generate an exception //in this block } catch( <exceptionType1> <varName1> ){ //write the action your program will do if an exception //of a certain type occurs } . . . catch( <exceptionTypen> <varNamen> ){ //write the action your program will do if an //exception of a certain type occurs } finally{ //add more cleanup code here } Introduction to Programming I 186
- 187. J.E.D.I Exceptions thrown during execution of the try block can be caught and handled in a catch block. The code in the finally block is always executed. The following are the key aspects about the syntax of the try-catch-finally construct: • The block notation is mandatory. • For each try block, there can be one or more catch blocks, but only one finally block. • The catch blocks and finally blocks must always appear in conjunction with the try block, and in the above order. • A try block must be followed by at least one catch block OR one finally block, or both. • Each catch block defines an exception handle. The header of the catch block takes exactly one argument, which is the exception its block is willing to handle. The exception must be of the Throwable class or one of its subclasses. Let's take for example a code that prints the second argument when we try to run the code using command-line arguments. Suppose, there is no checking inside your code for the number of arguments and we just access the second argument args[1] right away, we'll get the following exception. Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1 at ExceptionExample.main(ExceptionExample.java:5) Introduction to Programming I 187 Figure 12.1: Flow of events in a try-catch-finally block
- 188. J.E.D.I To prevent this from happening, we can place the code inside a try-catch block. The finally block is just optional. For this example, we won't use the finally block. public class ExceptionExample { public static void main( String[] args ){ try{ System.out.println( args[1] ); }catch( ArrayIndexOutOfBoundsException exp ){ System.out.println("Exception caught!"); } } } So when we try to run the program again without arguments, the output would be, Exception caught! Introduction to Programming I 188
- 189. J.E.D.I 12.4 Exercises 12.4.1 Catching Exceptions1 Given the following code: public class TestExceptions{ public static void main( String[] args ){ for( int i=0; true; i++ ){ System.out.println("args["+i+"]="+ args[i]); } } } Compile and run the TestExceptions program. The output should look like this: javac TestExceptions one two three args[0]=one args[1]=two args[2]=three Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3 at TestExceptions.main(1.java:4) Modify the TestExceptions program to handle the exception. The output of the program after catching the exception should look like this: javac TestExceptions one two three args[0]=one args[1]=two args[2]=three Exception caught: java.lang.ArrayIndexOutOfBoundsException: 3 Quiting... 12.4.2 Catching Exceptions 2 Chances are very good that some programs you've written before have encountered exceptions. Since you didn't catch the exceptions, they simply halted the execution of your code. Go back to those programs and implement exception handling. Introduction to Programming I 189
- 190. J.E.D.I Appendix A : Java and NetBeans Installation In this section, we will discuss on how to install Java and NetBeans in your system (Ubuntu Dapper/Windows XP). If you are not provided with the Java 5.0 and NetBeans 5.5 installers by your instructor, you can download a copy of the installers from the Sun Microsystems website (http://java.sun.com/) for Java and http://www.NetBeans.org/downloads/ for NetBeans). Before starting with the installation, copy the installers in your hard disk first. For Ubuntu Dapper: Copy all the installers inside the /usr folder. For Windows: Just copy the installers in any temporary directory. Introduction to Programming I 190
- 191. J.E.D.I Installing Java in Ubuntu Dapper Step 1: Go to the folder where you have your installers Step 2: Before running your installer, make sure it is executable. To do this, right click on the installer, click on the Permissions tab, and then Click on the execute box. Close the window. Introduction to Programming I 191
- 192. J.E.D.I Step 3: Double click on the file jdk-1_5_0_06-linux-i586.bin. A dialog box will display, click on the button 'Run In Terminal'. After pressing ENTER, you will see the license agreement displayed on the console. Introduction to Programming I 192
- 193. J.E.D.I Just press enter, until you see the question: Do you agree to the above license terms? [yes or no]. Just type: yes and press ENTER. Just wait for the installer to finish unpacking all its contents and installing Java. Introduction to Programming I 193
- 194. J.E.D.I Step 4: Creating symbolic links In order to run java commands anywhere, we need to create symbolic links for all the commands in JDK inside the /usr/local/bin directory. To do this, go to the directory:/usr/local/bin. Type: cd /usr/local/bin Introduction to Programming I 194
- 195. J.E.D.I To make the symbolic links to the commands, type: sudo ln -s /usr/java/jdk1.5.0_01/bin/* . Introduction to Programming I 195
- 196. J.E.D.I Installing Java in Windows Step 1: Using Windows Explorer, go to the folder where your Java installer is located Introduction to Programming I 196 Figure 12.2: Folder containing installers
- 197. J.E.D.I Introduction to Programming I 197
- 198. J.E.D.I Step 2: Run the installer To run the installer, just double-click on the installer icon. A J2SE installer dialog will then appear. Click on the radio button labeled "I accept the terms in the license agreement" and press NEXT. Introduction to Programming I 198 Figure 12.3: License agreement
- 199. J.E.D.I Click on NEXT to continue installation. Click on FINISH to complete installation. Introduction to Programming I 199 Figure 12.4: Custom setup Figure 12.5: Finish installation
- 200. J.E.D.I Installing NetBeans in Ubuntu Dapper Step 1: Go to the folder where you have your installers Introduction to Programming I 200
- 201. J.E.D.I Step 2: Before running your installer, make sure it is executable. To do this, right click on the installer, click on the Permissions tab, and then Click on the execute box. Close the window. Step 3: Double click on the file netbeans-5_5-beta-linux.bin. Click on Run in Terminal. Introduction to Programming I 201
- 202. J.E.D.I A Netbeans 5.5 dialog will then appear. Click on NEXT. Introduction to Programming I 202
- 203. J.E.D.I Click on the radio button that says "I accept the terms in the license agreement". And then click on NEXT. For the directory name, change it to: /usr/java/netbeans-4.0, then click on NEXT. Introduction to Programming I 203
- 204. J.E.D.I For the JDK directory, choose /usr/java/jdk1.5.0_01, and then click on NEXT. The next dialog just shows information about NetBeans thatyou will install. Just click again on NEXT. Introduction to Programming I 204
- 205. J.E.D.I Now, just wait for NetBeans to finish its installation. Click on FINISH to complete the installation. Introduction to Programming I 205
- 206. J.E.D.I Introduction to Programming I 206
- 207. J.E.D.I Step 4: Creating symbolic links In order to run NetBeans anywhere, we need to create symbolic link for it. To do this, go first to the directory:/usr/local/bin. Type: cd /usr/local/bin Make a symbolic link to the NetBeans executable by typing: sudo ln -s /home/florence/netbeans/bin/netbeans . Now, you can run NetBeans in any directory by typing: netbeans & Introduction to Programming I 207
- 208. J.E.D.I Installing NetBeans in Windows Step 1: Using Windows Explorer, go to the folder where your NetBeans installer is located Introduction to Programming I 208 Figure 12.6: NetBeans installer file
- 209. J.E.D.I Step 2: Run the installer To run the installer, just double-click on the installer icon. After clicking on the netbeans- 5_5-beta-bin-windows icon, the NetBeans installation wizard will appear. Click on NEXT to enter installation process. Introduction to Programming I 209 Figure 12.7: NetBeans installation
- 210. J.E.D.I The agreement page will the appear. Choose to ACCEPT and click NEXT to continue. Introduction to Programming I 210 Figure 12.8: License Agreement
- 211. J.E.D.I Then you will be given the choice on which directory to place the NetBeans. You can move on by clicking NEXT or you can click on BROWSE to choose a different directory. Introduction to Programming I 211 Figure 12.9: Choose directory where to install NetBeans
- 212. J.E.D.I Next is choosing the Standard Edition JDKs from your machine. If you have finished installing Java, the jdk1.5.0_01 chould appear from your choices. Click on NEXT to continue. Introduction to Programming I 212 Figure 12.10: Choose JDK to use
- 213. J.E.D.I It will then inform you the location and size of NetBeans which will be installed to your machine. Click on NEXT to finish installation. Introduction to Programming I 213 Figure 12.11: Installation Summary
- 214. J.E.D.I You have installed NetBeans on your computer. Click on FINISH to complete installation. Introduction to Programming I 214 Figure 12.12: Successful installation
- 215. J.E.D.I Appendix B: Getting to know your Programming Environment (Windows XP version) In this section, we will be discussing on how to write, compile and run Java programs. There are two ways of doing this, the first one is by using a console and a text editor. The second one is by using NetBeans which is an Integrated Development Environment or IDE. An IDE is a programming environment integrated into a software application that provides a GUI builder, a text or code editor, a compiler and/or interpreter and a debugger. Before going into details, let us first take a look at the first Java program you will be writing. My First Java Program public class Hello { /** * My first java program */ public static void main(String[] args) { //prints the string "Hello world" on screen System.out.println("Hello world!"); } } Before we try to explain what the program means, let's first try to write this program in a file and try to run it. Introduction to Programming I 215
- 216. J.E.D.I Using a Text Editor and Console For this example, we will be using the text editor "Notepad"(for Windows) to edit the Java source code. You can use other text editors if you want to. You will also need to open the MS-DOS prompt window to compile and execute your Java programs. Step 1: Start Notepad To start Notepad in Windows, click on start-> All Programs-> Accessories-> Notepad. Introduction to Programming I 216 Figure 12.14: Notepad Application Figure 12.13: Click on start-> All Programs-> Accessories -> Notepad
- 217. J.E.D.I Step 2: Open the Command Prompt window To open the MSDOS command prompt in Windows, click on start-> All programs-> Accessories-> Command Prompt. Introduction to Programming I 217 Figure 12.15: start-> All programs-> Accessories -> Command Prompt Figure 12.16: MSDOS Command Prompt
- 218. J.E.D.I Step 3: Write your the source code of your Java program in Notepad Step 4: Save your Java Program We will save our program on a file named "Hello.java", and we will be saving it inside a folder named MYJAVAPROGRAMS. To open the Save dialog box, click on the File menu found on the menubar and then click on Save. Introduction to Programming I 218
- 219. J.E.D.I After doing the procedure described above, a dialog box will appear as shown in Figure below. Click on the MY DOCUMENTS button to open the My Documents folder where we will be saving all your Java programs. Introduction to Programming I 219 Figure 12.17: This Dialog appears after clicking on File -> Save
- 220. J.E.D.I Now, we'll create a new folder inside the My Documents folder where we will save your programs. We shall name this folder MYJAVAPROGRAMS. Click on the button encircled in the figure below to create the folder. Introduction to Programming I 220 Figure 12.18: Click on the button encircled. This will open your "My Documents" folder Figure 12.19: Clicking on the encircled button will create a New Folder.
- 221. J.E.D.I After the folder is created, you can type in the desired name for this folder. In this case, type in MYJAVAPROGRAMS, and then press ENTER. Introduction to Programming I 221
- 222. J.E.D.I Now that we've created the folder where we will save all the files, double click on that folder to open it. You will see a similar figure as shown below. The folder should be empty for now since it's a newly created folder and we haven't saved anything in it yet. Introduction to Programming I 222
- 223. J.E.D.I Now click on the drop down list box "Save as type", so that we can choose what kind of file we want to save. Click on the "All Files" option. Now, in the Filename textbox, type in the filename of your program, which is "Hello.java", and then click on the SAVE button. Introduction to Programming I 223
- 224. J.E.D.I Now that you've saved your file, notice how the title of the frame changes from Untitled- Notepad to Hello.java-Notepad. Take note that if you want to make changes in your file, you can just edit it, and then save it again by clicking on File -> Save. Introduction to Programming I 224
- 225. J.E.D.I Step 5: Compiling your program Now, the next step is to compile your program. Go to the MSDOS command prompt window we just opened a while ago. Typically, when you open the command prompt window, it opens up and takes you directly to what is called your home folder. To see what is inside that home folder, type DIR or dir and then press ENTER. What you will see is a list of files and folders inside your home folder. Now, you can see here that there is a folder named "My Documents" where we created your MYJAVAPROGRAMS folder. Now let's go inside that directory. Introduction to Programming I 225 Figure 12.20: List of files and folders shown after executing the command DIR.
- 226. J.E.D.I To go inside a directory, you type in the command: cd [directory name]. The "cd" command stands for, change directory. In this case, since the name of our directory is My Documents, you type in: cd My Documents. Now that you are inside the "My Documents" folder, try typing in the "dir" command again, and tell me what you see. Now perform the same steps described before to go inside the MYJAVAPROGRAMS folder. Introduction to Programming I 226 Figure 12.21: Inside the My Documents folder Figure 12.22: The contents of My Documents
- 227. J.E.D.I Once inside the folder where your Java programs are, let us now start compiling your Java program. Take note that, you should make sure that the file is inside the folder where you are in. In order to do that, execute the dir command again to see if your file is inside that folder. To compile a Java program, we type in the command: javac [filename]. So in this case, type in: javac Hello.java. During compilation, javac adds a file to the disk called [filename].class, or in this case, Hello.class, which is the actual bytecode. Introduction to Programming I 227 Figure 12.23: Inside the MYJAVAPROGRAMS folder Figure 12.24: Compile program by usingthe javac command
- 228. J.E.D.I Step 6: Running the Program Now, assuming that there are no problems during compilation (we'll explore more of the problems encountered during compilation in the next section), we are now ready to run your program. To run your Java program, type in the command: java [filename without the extension], so in the case of our example, type in: java Hello You can see on the screen that you have just run your first Java program that prints the message, "Hello world!". Introduction to Programming I 228 Figure 12.25: Output of the program
- 229. J.E.D.I Setting the Path Sometimes, when you try to invoke the javac or java command, you encounter the message: 'javac' is not recognized as an internal or external command, operable program or batch file. This means that either you haven't installed Java in your system yet, or you have to configure the path on where the Java commands are installed so that your system will know where to find them. If you are sure that you've already installed Java in your system, try setting the PATH variable to point to where the Java commands are installed. To do this, type in the command: set PATH=C:j2sdk1.4.2_04bin. This will tell your system to look for the commands in the C:j2sdk1.4.2_04bin folder, which is usually the default location wherein your Java files are placed during installation. After doing this, you can now use the Java commands. Introduction to Programming I 229 Figure 12.26: System did not recognize the javac command Figure 12.27: Setting the path and running java
- 230. J.E.D.I Using NetBeans Now that we've tried doing our programs the complicated way, let's now see how to do all the processes we've described in the previous sections by using just one application. In this part of the lesson, we will be using NetBeans, which is an Integrated Development Environment or IDE. An IDE is a programming environment integrated into a software application that provides a GUI builder, a text or code editor, a compiler and/or interpreter and a debugger. Step 1: Run NetBeans To run NetBeans, click on start-> All Programs-> NetBeans 5.5 Beta -> NetBeans IDE Introduction to Programming I 230
- 231. J.E.D.I After you've open NetBeans IDE, you will see a graphical user interface (GUI) similar to what is shown below. Introduction to Programming I 231 Figure 12.28: NetBeans IDE
- 232. J.E.D.I Step 2: Make a project Now, let's first make a project. Click on File-> New Project. After doing this, a New Project dialog will appear. Introduction to Programming I 232
- 233. J.E.D.I Now click on Java Application and click on the NEXT button. Introduction to Programming I 233
- 234. J.E.D.I Now, a New Application dialog will appear. Edit the Project Name part and type in "HelloApplication". Introduction to Programming I 234 Figure 12.29: Change Project Name
- 235. J.E.D.I Now try to change the Application Location, by clicking on the BROWSE button. Follow the steps described in the previous section to go to your MYJAVAPROGRAMS folder. Finally, on the Create Main Class textfield, type in Hello as the main class' name, and then click on the FINISH button. Introduction to Programming I 235
- 236. J.E.D.I Step 3: Type in your program Before typing in your program, let us first describe the main window after creating the project. As shown below, NetBeans automatically creates the basic code for your Java program. You can just add your own statements to the generated code. On the left side of the window, you can see a list of folders and files that NetBeans generated after creating the project. This can all be found in your MYJAVAPROGRAMS folder, where you set the Project location. Now, try to modify the code generated by NetBeans. Ignore the other parts of the program for now, as we will explain the details of the code later. Insert the code: System.out.println("Hello world!"); after the statement, //TODO code application logic here. Introduction to Programming I 236
- 237. J.E.D.I Step 4: Compile your program Now, to compile your program, just click on Build -> Build Main Project. Or, you could also use the shortcut button to compile your code. Introduction to Programming I 237 Figure 12.30: Shortcut button to compile code
- 238. J.E.D.I If there are no errors in your program, you will see a build successful message on the output window. Introduction to Programming I 238 Figure 12.31: Output window just below the window where you type your source code
- 239. J.E.D.I Step 5: Run your program To run your program, click on Run-> Run Main Project. Or you could also use the shortcut button to run your program. The output of your program is displayed in the output window. Introduction to Programming I 239 Figure 12.32: Shortcut button to run program Figure 12.33: Output of Hello.java
- 240. J.E.D.I Appendix D : Machine Problems Machine Problem 1: Phone Book Write a program that will create an phonebook, wherein you can add entries in the phonebook, delete entries, view all entries and search for entries. In viewing all entries, the user should have a choice, whether to view the entries in alphabetical order or in increasing order of telephone numbers. In searching for entries, the user should also have an option to search entries by name or by telephone numbers. In searching by name, the user should also have an option if he/she wants to search by first name or last name. MAIN MENU 1 - Add phonebook entry 2 - Delete phonebook entry 3 - View all entries a - alphabetical order b - increasing order of telephone numbers 4 - Search entries a - by name b - by telephone number 5 – Quit The following will appear when one of the choices in the main menu is chosen. Add phonebook entry Enter Name: Enter Telephone number: (* if entry already exists, warn user about this) View all entries Displays all entries in alphabetical order Displays all entries in increasing order of telephone #s Search entries Search phonebook entry by name Search phonebook entry by telephone number Quit close phonebook Introduction to Programming I 240
- 241. J.E.D.I Machine Problem 2: Minesweeper This is a one player game of a simplified version of the popular computer game minesweeper. First, the user is asked if he or she wants to play on a 5x5 grid or 10x10 grid. You have two 2-dimensional arrays that contains information about your grid. An entry in the array should either contain a 0 or 1. A 1 signifies that there is a bomb in that location, and a 0 if none. For example, given the array: int bombList5by5[][]={{0, 0, 1, 0, 0}, {0, 0, 0, 0, 0}, {0, 1, 0, 0, 0}, {0, 0, 0, 1, 1}, {0, 1, 1, 0, 0}}; Given the bomb list, we have 6 bombs on our list. The bombs are located in (row,col) cells, (0,2), (2,1), (3,3), (3,4), (4,1) and (4,2). If the user chooses a cell that contains a bomb, the game ends and all the bombs are displayed. If the user chooses a cell that does not contain a bomb, a number appears at that location indicating the number of neighbors that contain bombs. The game should end when all the cells that do not contain bombs have been marked (player wins) or when the user steps on a bomb(player loses). Here's a sample output of the game, given the bombList5by5. Welcome to Minesweeper! Choose size of grid (Press 1 for 5x5, Press 2 for 10x10): 1 [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] Enter row and column of the cell you want to open[row col]: 1 1 [ ] [ ] [ ] [ ] [ ] [ ] [2] [ ] [ ] [ ] [ ] [ ] [] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] Enter row and column of the cell you want to open[row col]: 3 2 [ ] [ ] [ ] [ ] [ ] [ ] [2 ] [ ] [] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [4 ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] Enter row and column of the cell you want to open[row col]: 0 2 [] [ ] [ ] [ ] [ ] [ ] [2] [ ] [] [ ] [ ] [X ] [ ] [ ] [ ] [ ] [ ] [4] [ ] [ ] [ ] [ ] [ ] [ ] [ ] Ooppps! You stepped on a bomb. Sorry, game over! Introduction to Programming I 241
- 242. J.E.D.I Machine Problem 3: Number Conversion Create your own scientific calculator that will convert the inputted numbers to the four number representations ( Decimal, Binary, Octal, Hexadecimal ). Your program should output the following menu on screen. MAIN MENU: Please type the number of your choice: 1 – Binary to Decimal 2 – Decimal to Octal 3 – Octal to Hexadecimal 4 – Hexadecimal to Binary 5 – Quit The following will appear when one of the choices in the main menu is chosen. Choice 1: Enter a binary number: 11000 11000 base 2 = 24 base 10 (goes back to main menu) Choice 2: Enter a Decimal number: 24 24 base 10 = 30 base 8 (goes back to main menu) Choice 3: Enter an Octal number: 30 30 base 8 = 18 base 16 (goes back to main menu) Choice 4: Enter a Hexadecimal number: 18 18 base 16 = 11000 base 2 Choice 1: Enter a binary number: 110A Invalid binary number! Enter a binary number: 1 1 base 2 = 1 base 10 (goes back to main menu) (user chooses 5) Goodbye! You can be more creative with your user interface if you want to, as long as the program outputs the correct conversion of numbers. Introduction to Programming I 242
- 243. J.E.D.I References 1. Programming Language. From Wikipedia at http://en.wikipedia.org/wiki/Programming_language 2. Programming Language. From Webopedia at http://www.webopedia.com/TERM/p/programming_language.html 3. Programming Language. From Answers.com at http://www.answers.com/topic/programming-language 4. High-Level Programming Language. From Wikipedia at http://en.wikipedia.org/wiki/High-level_programming_language 5. Defining Flowchart Symbols. Available at http://www.patton- patton.com/basic_flow_chart_symbols.htm 6. Integrated Development Environment. From Webopedia at http://www.webopedia.com/TERM/I/integrated_development_environment.html 7. Variables and Expressions. Available at http://www.geocities.com/SiliconValley/Park/3230/java/javl1002.html 8. Writing Abstract Classes and Methods. Available at http://java.sun.com/docs/books/tutorial/java/javaOO/abstract.html 9. Defining an Interface. Available at http://java.sun.com/docs/books/tutorial/java/interpack/interfaceDef.html 20.Inheritance and Polymorphism. Available at http://home.cogeco.ca/~ve3ll/jatutor7.htm 21.The Essence of OOP using Java, Runtime Polymorphism through Inheritance. Available at http://www.developer.com/tech/article.php/983081 22.Gary B. Shelly, Thomas J. Cashman, Joy L. Starks. Java Programming Complete Concepts and Techniques. Course Technology Thomson Learning. 2001. 23.Stephen J. Chapman. Java for Engineers and Scientists 2nd Edition. Pearson Prentice Hall. 2004 24.Deitel & Deitel. Java How to Program 5th Edition. 25.Sun Java Programming Student Guide SL-275. Sun Microsystems. February 2001. 26.Does Java pass by reference or pass by value? Why can't you swap in Java? Available at http://www.javaworld.com/javaworld/javaqa/2000-05/03-qa-0526-pass.html 27.Java Branching Statements. Available at http://java.sun.com/docs/books/tutorial/java/nutsandbolts/branch.html. 28.Encapsulation. Available at http://home.cogeco.ca/~ve3ll/jatutor4.htm. Introduction to Programming I 243