Java™ (OOP) - Chapter 2: "Elementary Programming"

Chapter 2 Elementary Programming

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807

1

Motivations
In the preceding chapter, you learned how to
create, compile, and run a Java program. Starting
from this chapter, you will learn how to solve
practical problems programmatically. Through
these problems, you will learn Java primitive data
types and related subjects, such as variables,
constants, data types, operators, expressions, and
input and output.


2

Objectives
To write Java programs to perform simple calculations (§2.2).
To obtain input from the console using the Scanner class (§2.3).
To use identifiers to name variables, constants, methods, and classes (§2.4).
To use variables to store data (§§2.5-2.6).
To program with assignment statements and assignment expressions (§2.6).
To use constants to store permanent data (§2.7).
To declare Java primitive data types: byte, short, int, long, float, double, and char
(§§2.8.1).
To use Java operators to write numeric expressions (§§2.8.2–2.8.3).
To display current time (§2.9).
To use short hand operators (§2.10).
To cast value of one type to another type (§2.11).
To compute loan payment (§2.12).
To represent characters using the char type (§2.13).
To compute monetary changes (§2.14).
To represent a string using the String type (§2.15).
To become familiar with Java documentation, programming style, and naming
conventions (§2.16).
To distinguish syntax errors, runtime errors, and logic errors and debug errors (§2.17).
(GUI) To obtain input using the JOptionPane input dialog boxes (§2.18).

3

Introducing Programming with an
Example
Listing 2.1 Computing the Area of a
Circle
This program computes the area of the
circle.
ComputeArea IMPORTANT NOTE: To enable the buttons, you must
Run

download the entire slide file slide.zip and unzip the files
into a directory (e.g., c:slide) .


4

animation

Trace a Program Execution

public class ComputeArea {
/** Main method */
public static void main(String[] args) {
double radius;
double area;

allocate memory
for radius
radius

no value

// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}


5

animation


/** Main method */
double radius;
double area;
// Assign a radius
radius = 20;
// Compute area

memory
radius

no value

area

no value

allocate memory
for area

// Display results
}
}


6

animation


/** Main method */
double radius;
double area;

assign 20 to radius
radius
area

20
no value

// Assign a radius
radius = 20;
// Compute area
// Display results
}
}


7

animation


/** Main method */
double radius;
double area;

memory
radius
area

20
1256.636

// Assign a radius
radius = 20;
// Compute area

compute area and assign
it to variable area

// Display results
}
}


8

animation


/** Main method */
double radius;
double area;

memory
radius
area

20
1256.636

// Assign a radius
radius = 20;
// Compute area

print a message to the
console

// Display results
}
}


9

Reading Input from the Console
1. Create a Scanner object
Scanner input = new Scanner(System.in);

2. Use the methods next(), nextByte(), nextShort(),
nextInt(), nextLong(), nextFloat(), nextDouble(), or
nextBoolean() to obtain to a string, byte, short, int,
long, float, double, or boolean value. For example,
System.out.print("Enter a double value: ");
Scanner input = new Scanner(System.in);
double d = input.nextDouble();
ComputeAreaWithConsoleInput
Run

ComputeAverage
Run


10

Identifiers
An identifier is a sequence of characters that consist of
letters, digits, underscores (_), and dollar signs ($).
An identifier must start with a letter, an underscore (_),
or a dollar sign ($). It cannot start with a digit.
– An identifier cannot be a reserved word. (See Appendix A,
“Java Keywords,” for a list of reserved words).

An identifier cannot be true, false, or
null.
An identifier can be of any length.


11

Variables
// Compute the first area
radius = 1.0;
System.out.println("The area is “ +
area + " for radius "+radius);
// Compute the second area
radius = 2.0;
System.out.println("The area is “ +
area + " for radius "+radius);

12

Declaring Variables
int x;

// Declare x to be an
// integer variable;

double radius; // Declare radius to
// be a double variable;
char a;

// Declare a to be a
// character variable;


13

Assignment Statements
x = 1;

// Assign 1 to x;

radius = 1.0;

// Assign 1.0 to radius;

a = 'A';

// Assign 'A' to a;


14

Declaring and Initializing
in One Step
int x = 1;
double d = 1.4;


15

Constants
final datatype CONSTANTNAME = VALUE;
final double PI = 3.14159;
final int SIZE = 3;


16

Numerical Data Types
Name

Range

Storage Size

byte

–27 (-128) to 27–1 (127)

8-bit signed

short

–215 (-32768) to 215–1 (32767)

16-bit signed

int

–231 (-2147483648) to 231–1 (2147483647) 32-bit signed

long

–263 to 263–1
(i.e., -9223372036854775808
to 9223372036854775807)

64-bit signed

float

Negative range:
-3.4028235E+38 to -1.4E-45
Positive range:
1.4E-45 to 3.4028235E+38

32-bit IEEE 754

double

Negative range:
-1.7976931348623157E+308 to
-4.9E-324
Positive range:
4.9E-324 to 1.7976931348623157E+308

64-bit IEEE 754


17

Numeric Operators
Name

Meaning

Example

Result

+

Addition

34 + 1

35

-

Subtraction

34.0 – 0.1

33.9

*

Multiplication

300 * 30

9000

/

Division

1.0 / 2.0

0.5

%

Remainder

20 % 3

2


18

Integer Division
+, -, *, /, and %
5 / 2 yields an integer 2.
5.0 / 2 yields a double value 2.5
5 % 2 yields 1 (the remainder of the division)


19

Remainder Operator
Remainder is very useful in programming. For example, an
even number % 2 is always 0 and an odd number % 2 is always
1. So you can use this property to determine whether a number
is even or odd. Suppose today is Saturday and you and your

friends are going to meet in 10 days. What day is in 10
days? You can find that day is Tuesday using the following
expression:
Saturday is the 6th day in a week
A week has 7 days
(6 + 10) % 7 is 2

The 2nd day in a week is Tuesday

After 10 days


20

Problem: Displaying Time
Write a program that obtains hours and
minutes from seconds.

DisplayTime

Run


21

NOTE
Calculations involving floating-point numbers are
approximated because these numbers are not stored
with complete accuracy. For example,
System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);
displays 0.5000000000000001, not 0.5, and
System.out.println(1.0 - 0.9);
displays 0.09999999999999998, not 0.1. Integers are
stored precisely. Therefore, calculations with integers
yield a precise integer result.

22

Number Literals
A literal is a constant value that appears directly
in the program. For example, 34, 1,000,000, and
5.0 are literals in the following statements:
int i = 34;
long x = 1000000;
double d = 5.0;


23

Integer Literals
An integer literal can be assigned to an integer variable as
long as it can fit into the variable. A compilation error
would occur if the literal were too large for the variable to
hold. For example, the statement byte b = 1000 would
cause a compilation error, because 1000 cannot be stored
in a variable of the byte type.
An integer literal is assumed to be of the int type, whose
value is between -231 (-2147483648) to 231–1 (2147483647).
To denote an integer literal of the long type, append it with
the letter L or l. L is preferred because l (lowercase L) can
easily be confused with 1 (the digit one).

24

Floating-Point Literals
Floating-point literals are written with a decimal
point. By default, a floating-point literal is treated
as a double type value. For example, 5.0 is
considered a double value, not a float value. You
can make a number a float by appending the letter f
or F, and make a number a double by appending
the letter d or D. For example, you can use 100.2f
or 100.2F for a float number, and 100.2d or 100.2D
for a double number.

25

Scientific Notation
Floating-point literals can also be specified in
scientific notation, for example, 1.23456e+2,
same as 1.23456e2, is equivalent to 123.456, and
1.23456e-2 is equivalent to 0.0123456. E (or e)
represents an exponent and it can be either in
lowercase or uppercase.


26

Arithmetic Expressions
3 + 4 x 10( y − 5)( a + b + c )
4 9+ x
−
+ 9( +
)
5
x
x
y

is translated to

(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)


27

How to Evaluate an Expression
Though Java has its own way to evaluate an
expression behind the scene, the result of a Java
expression and its corresponding arithmetic expression
are the same. Therefore, you can safely apply the
arithmetic rule for evaluating a Java expression.
3 + 4 * 4 + 5 * (4 + 3) - 1
3 + 4 * 4 + 5 * 7 – 1
3 + 16 + 5 * 7 – 1

(1) inside parentheses first
(2) multiplication
(3) multiplication

3 + 16 + 35 – 1
19 + 35 – 1
54 - 1
53

(4) addition
(5) addition
(6) subtraction


28

Problem: Converting Temperatures
Write a program that converts a Fahrenheit degree
to Celsius using the formula:
5
celsius = ( 9 )( fahrenheit − 32)

FahrenheitToCelsius

Run


29

Problem: Displaying Current Time
Write a program that displays current time in GMT in the
format hour:minute:second such as 1:45:19.
The currentTimeMillis method in the System class returns
the current time in milliseconds since the midnight, January
1, 1970 GMT. (1970 was the year when the Unix operating
system was formally introduced.) You can use this method
to obtain the current time, and then compute the current
second, minute, and hour as follows.
ShowCurrentTime

Elapsed
time
Unix Epoch
01-01-1970
00:00:00 GMT

Time
Current Time

System.currentTimeMills()


Run
30

Shortcut Assignment Operators
Operator Example

Equivalent

+=

i += 8

i = i + 8

-=

f -= 8.0

f = f - 8.0

*=

i *= 8

i = i * 8

/=

i /= 8

i = i / 8

%=

i %= 8

i = i % 8


31

Increment and
Decrement Operators
Operator
++var

Name
preincrement

var++

postincrement

--var

predecrement

var--

postdecrement

Description
The expression (++var) increments var by 1 and evaluates
to the new value in var after the increment.
The expression (var++) evaluates to the original value
in var and increments var by 1.
The expression (--var) decrements var by 1 and evaluates
to the new value in var after the decrement.
The expression (var--) evaluates to the original value
in var and decrements var by 1.


32

Increment and
Decrement Operators, cont.
int i = 10;
int newNum = 10 * i++;

Same effect as

int i = 10;
int newNum = 10 * (++i);

int newNum = 10 * i;
i = i + 1;

Same effect as

i = i + 1;
int newNum = 10 * i;


33

Increment and
Decrement Operators, cont.
Using increment and decrement operators makes
expressions short, but it also makes them complex and
difficult to read. Avoid using these operators in
expressions that modify multiple variables, or the same
variable for multiple times such as this: int k = ++i + i.


34

Assignment Expressions and
Assignment Statements
Prior to Java 2, all the expressions can be used as
statements. Since Java 2, only the following types of
expressions can be statements:
variable op= expression; // Where op is +, -, *, /, or %
++variable;
variable++;
--variable;
variable--;

35

Numeric Type Conversion
Consider the following statements:
byte i = 100;
long k = i * 3 + 4;
double d = i * 3.1 + k / 2;


36

Conversion Rules
When performing a binary operation involving two
operands of different types, Java automatically
converts the operand based on the following rules:
1. If one of the operands is double, the other is
converted into double.
2. Otherwise, if one of the operands is float, the other is
converted into float.
3. Otherwise, if one of the operands is long, the other is
converted into long.
4. Otherwise, both operands are converted into int.

37

Type Casting
Implicit casting
double d = 3; (type widening)
Explicit casting
int i = (int)3.0; (type narrowing)
int i = (int)3.9; (Fraction part is
truncated)
What is wrong?
int x = 5 / 2.0;
range increases
byte, short, int, long, float, double


38

Problem: Keeping Two Digits After
Decimal Points
Write a program that displays the sales tax with two
digits after the decimal point.

SalesTax

Run


39

Problem:
Computing Loan Payments
This program lets the user enter the interest
rate, number of years, and loan amount and
computes monthly payment and total
payment.
loanAmount × monthlyInterestRate
monthlyPayment =
1
1−
(1 + monthlyInterestRate) numberOfYears×12
ComputeLoan

Run


40

Character Data Type
char letter = 'A'; (ASCII)
char numChar = '4'; (ASCII)

Four hexadecimal digits.

char letter = 'u0041'; (Unicode)
char numChar = 'u0034'; (Unicode)
NOTE: The increment and decrement operators can also be used on char
variables to get the next or preceding Unicode character. For example,
the following statements display character b.
char ch = 'a';
System.out.println(++ch);

41

Unicode Format
Java characters use Unicode, a 16-bit encoding scheme
established by the Unicode Consortium to support the
interchange, processing, and display of written texts in the
world’s diverse languages. Unicode takes two bytes,
preceded by u, expressed in four hexadecimal numbers
that run from 'u0000' to 'uFFFF'. So, Unicode can
represent 65535 + 1 characters.
Unicode u03b1 u03b2 u03b3 for three Greek
letters


42

Problem: Displaying Unicodes
Write a program that displays two Chinese
characters and three Greek letters.

DisplayUnicode

Run


43

Escape Sequences for Special Characters
Description

Escape Sequence

Unicode

Backspace

b

u0008

Tab

t

u0009

Linefeed

n

u000A

Carriage return r

u000D

Backslash

u005C

Single Quote

'

u0027

Double Quote

"

u0022


44

Appendix B: ASCII Character Set
ASCII Character Set is a subset of the Unicode from u0000 to u007f


45

ASCII Character Set, cont.
ASCII Character Set is a subset of the Unicode from u0000 to u007f


46

Casting between char and
Numeric Types
int i = 'a'; // Same as int i = (int)'a';
char c = 97; // Same as char c = (char)97;


47

Problem: Monetary Units
This program lets the user enter the amount in
decimal representing dollars and cents and output
a report listing the monetary equivalent in single
dollars, quarters, dimes, nickels, and pennies.
Your program should report maximum number of
dollars, then the maximum number of quarters,
and so on, in this order.
ComputeChange

Run
48

Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100);
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount = remainingAmount % 100;

remainingAmount

1156

remainingAmount
initialized

// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;

49

animation

Trace ComputeChange


remainingAmount


numberOfOneDollars


1156
11

numberOfOneDollars
assigned


50

animation

Trace ComputeChange


remainingAmount

56


numberOfOneDollars

11


remainingAmount
updated


51

animation

Trace ComputeChange


remainingAmount

56


numberOfOneDollars

11


numberOfOneQuarters

2


numberOfOneQuarters
assigned


52

animation

Trace ComputeChange


remainingAmount

6


numberOfOneDollars

11


numberOfQuarters


2

remainingAmount
updated


53

The String Type
The char type only represents one character. To represent a string
of characters, use the data type called String. For example,
String message = "Welcome to Java";
String is actually a predefined class in the Java library just like the
System class and JOptionPane class. The String type is not a
primitive type. It is known as a reference type. Any Java class can
be used as a reference type for a variable. Reference data types
will be thoroughly discussed in Chapter 7, “Objects and Classes.”
For the time being, you just need to know how to declare a String
variable, how to assign a string to the variable, and how to
concatenate strings.

54

String Concatenation
// Three strings are concatenated
String message = "Welcome " + "to " + "Java";
// String Chapter is concatenated with number 2
String s = "Chapter" + 2; // s becomes Chapter2
// String Supplement is concatenated with character B
String s1 = "Supplement" + 'B'; // s1 becomes SupplementB


55

Programming Style and
Documentation
Appropriate Comments
Naming Conventions
Proper Indentation and Spacing
Lines
Block Styles


56

Appropriate Comments
Include a summary at the beginning of the
program to explain what the program does, its key
features, its supporting data structures, and any
unique techniques it uses.
Include your name, class section, instructor, date,
and a brief description at the beginning of the
program.


57

Naming Conventions
Choose meaningful and descriptive names.
Variables and method names:
– Use lowercase. If the name consists of several
words, concatenate all in one, use lowercase
for the first word, and capitalize the first letter
of each subsequent word in the name. For
example, the variables radius and area, and
the method computeArea.


58

Naming Conventions, cont.
Class names:
– Capitalize the first letter of each word in
the name. For example, the class name
ComputeArea.

Constants:
– Capitalize all letters in constants, and use
underscores to connect words. For
example, the constant PI and
MAX_VALUE


59

Proper Indentation and Spacing
Indentation
– Indent two spaces.

Spacing
– Use blank line to separate segments of the code.


60

Block Styles
Use end-of-line style for braces.
Next-line
style

public class Test
{
public static void main(String[] args)
{
System.out.println("Block Styles");
}
}

public class Test {
System.out.println("Block Styles");
}
}


End-of-line
style

61

Programming Errors
Syntax Errors
– Detected by the compiler

Runtime Errors
– Causes the program to abort

Logic Errors
– Produces incorrect result


62

Syntax Errors
public class ShowSyntaxErrors {
i = 30;
System.out.println(i + 4);
}
}


63

Runtime Errors
public class ShowRuntimeErrors {
int i = 1 / 0;
}
}


64

Logic Errors
public class ShowLogicErrors {
// Determine if a number is between 1 and 100 inclusively
// Prompt the user to enter a number
String input = JOptionPane.showInputDialog(null,
"Please enter an integer:",
"ShowLogicErrors", JOptionPane.QUESTION_MESSAGE);
int number = Integer.parseInt(input);
// Display the result
System.out.println("The number is between 1 and 100, " +
"inclusively? " + ((1 < number) && (number < 100)));
System.exit(0);
}
}


65

Debugging
Logic errors are called bugs. The process of finding and
correcting errors is called debugging. A common approach
to debugging is to use a combination of methods to narrow
down to the part of the program where the bug is located.
You can hand-trace the program (i.e., catch errors by
reading the program), or you can insert print statements in
order to show the values of the variables or the execution
flow of the program. This approach might work for a short,
simple program. But for a large, complex program, the
most effective approach for debugging is to use a debugger
utility.

66

Debugger
Debugger is a program that facilitates debugging.
You can use a debugger to
Execute a single statement at a time.
Trace into or stepping over a method.
Set breakpoints.
Display variables.
Display call stack.
Modify variables.


67

JOptionPane Input
This book provides two ways of obtaining input.
1.
2.

Using the Scanner class (console input)
Using JOptionPane input dialogs


68

Getting Input from Input Dialog Boxes
String input = JOptionPane.showInputDialog(
"Enter an input");


69

Getting Input from Input Dialog Boxes
String string = JOptionPane.showInputDialog(
null, “Prompting Message”, “Dialog Title”,
JOptionPane.QUESTION_MESSAGE);


70

Two Ways to Invoke the Method
There are several ways to use the showInputDialog method. For
the time being, you only need to know two ways to invoke it.
One is to use a statement as shown in the example:
String string = JOptionPane.showInputDialog(null, x,
y, JOptionPane.QUESTION_MESSAGE);

where x is a string for the prompting message, and y is a string for
the title of the input dialog box.
The other is to use a statement like this:
JOptionPane.showInputDialog(x);

where x is a string for the prompting message.

71

Converting Strings to Integers
The input returned from the input dialog box is a string. If
you enter a numeric value such as 123, it returns “123”.
To obtain the input as a number, you have to convert a
string into a number.
To convert a string into an int value, you can use the
static parseInt method in the Integer class as follows:
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as “123”.

72

Converting Strings to Doubles
To convert a string into a double value, you can use the
static parseDouble method in the Double class as follows:
double doubleValue =Double.parseDouble(doubleString);
where doubleString is a numeric string such as “123.45”.


73

Problem: Computing Loan Payments
Using Input Dialogs
Same as the preceding program for computing loan
payments, except that the input is entered from the
input dialogs and the output is displayed in an
output dialog.

loanAmount × monthlyInterestRate
1
1−
numberOfYears×12
(1 + monthlyInterestRate)
ComputeLoanUsingInputDialog

Run
74

Companion
Website

Debugging in NetBeans

Supplement II.E, Learning Java Effectively with
NetBeans


75

Companion
Website

Debugging in Eclipse

Supplement II.G, Learning Java Effectively with
NetBeans


76

Java™ (OOP) - Chapter 2: "Elementary Programming"

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