Ch3

2
Motivations
 In the preceding chapter, you learned
 how to create, compile, and run a Java
program.
 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.

3
Introducing Programming with an Example
Listing 2.1 Computing the Area of a Circle
public class ComputeArea {
public static void main(String[] args) {
double radius; // Declare radius
double area; // Declare area
radius = 20; // Assign a radius
// Compute area
area = radius * radius * 3.14159;
// Display results
System.out.println
("The area for the circle of radius " +
radius + " is " + area);
}
}

4
Trace a Program Execution
/** Main method */
double radius;
double area;
// Assign a radius
radius = 20;
// Compute area
// Display results
System.out.println("The area for the circle of radius " +
}
}
no valueradius
allocate memory
for radius
animation

5
/** Main method */
double radius;
double area;
// Assign a radius
radius = 20;
// Compute area
// Display results
}
}
no valueradius
memory
no valuearea
allocate memory
for area
animation

6
/** Main method */
double radius;
double area;
// Assign a radius
radius = 20;
// Compute area
// Display results
}
}
20radius
no valuearea
assign 20 to radius
animation

7
/** Main method */
double radius;
double area;
// Assign a radius
radius = 20;
// Compute area
// Display results
}
}
20radius
memory
1256.636area
compute area and assign
it to variable area
animation

8
/** Main method */
double radius;
double area;
// Assign a radius
radius = 20;
// Compute area
// Display results
}
}
20radius
memory
1256.636area
print a message to the
console
animation

9
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.
 Java is case sensitive
 Upper case letters are the same as lower case letters
 Examples
 Name is not the same as name

10
Variables
The area is 3.14159 for radius 1.0
The area is 12.56636 for radius 2.0
public class DisplayRadius {
public static void main(String[]args) {
double radius; //Declare variable radius
double area; //Declare variable area
// Compute the first area
radius = 1.0; //assign value to variable radius
area = radius * radius * 3.14159; //compute value of area
System.out.println("The area is " + area +
" for radius " + radius);
// Compute the second area
radius = 2.0; //assign value to variable radius
area = radius * radius * 3.14159; //compute value of area
System.out.println("The area is " + area +
" for radius "+radius);
}
}

11
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;
• Declarations are used to
• tell the compiler how much memory space is needed
for the value that will be assigned to the variable
• tell the compiler the type of data that will be
referenced by that variable

12
Assignment Statements
x = 1; // Assign 1 to x;
radius = 1.0; // Assign 1.0 to radius;
a = ‘D'; // Assign ‘D' to a;
• Assignment statements
• Assign a value to the variable
• That value is stored in the memory location
associated with that variable

13
Declaring and Initializing in One Step
 int x = 1;
 double d = 1.4;
• In Jave, you can declare a variable and store a value in
that variable in one statement
• It is usually a good idea to initialize the variable when it is
declared

14
Constants
 Constant
 Once a value is assigned it cannot be changed
 final is the keyword used to declare a variable a
constant
 Format
 final datatype CONSTANTNAME = VALUE;
 Examples
 final double PI = 3.14159;
 final int SIZE = 3;
 Note
 Convention is to use all upper case letters for the
name of the constant variable

15
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 64-bit signed
(i.e., -9223372036854775808
to 9223372036854775807)
float Negative range: 32-bit IEEE 754
-3.4028235E+38 to -1.4E-45
Positive range:
1.4E-45 to 3.4028235E+38
double Negative range: 64-bit IEEE 754
-1.7976931348623157E+308 to
-4.9E-324
Positive range:
4.9E-324 to 1.7976931348623157E+308

16
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

17
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)

18
Remainder Operator
 Remainder is very useful in programming.
 For example
 an even number % 2 is always 0
 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
After 10 days
The 2nd
day in a week is Tuesday
(6 + 10) % 7 is 2

19
Problem: Displaying Time
Write a program that obtains hours and
minutes from seconds.
public class DisplayTime {
int seconds = 500;
int minutes = seconds / 60;
int remainingSeconds = seconds % 60;
System.out.println(seconds + " seconds is " + minutes +
" minutes and " + remainingSeconds + " seconds");
}
}
500 seconds is 8 minutes and 20 seconds

20
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.

21
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;

22
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).

23
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
 You 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.

24
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.

25
Arithmetic Expressions
)
94
(9
))(5(10
5
43
y
x
xx
cbayx +
++
++−
−
+
is translated to
(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)

26
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
3 + 16 + 35 – 1
19 + 35 – 1
54 - 1
53
(1) inside parentheses first
(2) multiplication
(3) multiplication
(4) addition
(6) subtraction
(5) addition
Appendix C, page 690, has a
chart of operator precedence.

27
Problem: Converting Temperatures
Write a program that converts a Fahrenheit
degree to Celsius using the formula:
)32)((9
5
−= fahrenheitcelsius
public class FahrenheitToCelsius {
double fahrenheit = 435; // Say 100;
double celsius = (5.0 / 9) * (fahrenheit - 32);
System.out.println("Fahrenheit " + fahrenheit +
" is " + celsius + " in Celsius");
}
}
Fahrenheit 435.0 is 223.88888888888889 in Celsius

28
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

29
Increment and Decrement Operators
Operator Name Description
++var preincrement The expression (++var) increments var by 1 and evaluates
to the new value in var after the increment.
var++ postincrement The expression (var++) evaluates to the original value
in var and increments var by 1.
--var predecrement The expression (--var) decrements var by 1 and evaluates
to the new value in var after the decrement.
var-- postdecrement The expression (var--) evaluates to the original value
in var and decrements var by 1.
++var and --var: performs the arithmetic operation first than uses the value.
var++ and var– uses the value first than performs the arithmetic operation.

30
Increment and Decrement Operators, cont.
int i = 10;
int newNum = 10 * i++; int newNum = 10 * i;
i = i + 1;
Same effect as
int i = 10;
int newNum = 10 * (++i); i = i + 1;
int newNum = 10 * i;
Same effect as
newNum will have the value 100, i will have the value 11
newNum will have the value 110, i will have the value 11

31
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.

32
Assignment Expressions and Assignment Statements
Prior to Java 2, all the expressions could 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--;
• Expressions perform operations on data and move data around.
• All statements except blocks are terminated by a semicolon. Blocks are
denoted by open and close curly braces.

33
Numeric Type Conversion
Consider the following statements:
byte i = 100;
long k = i * 3 + 4;
double d = i * 3.1 + k / 2;
• Some of the statements have mixed types
• byte types are mixed with long types
• byte and long types are mixed with double type
byte type
long type

34
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.

35
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;
converts the 5 to 5.0, performs division to get 2.5 then tries to
store a double in an int, but it won’t fit. You will get a compile
error.
byte, short, int, long, float, double
range increases
8-bits 16-bits 32-bits 64-bits 32-bits 64-bits
Will store 3.000000000
Will store 3

36
Problem: Keeping Two Digits After Decimal Points
Write a program that displays the sales tax with
two digits after the decimal point.
public class SalesTax {
double purchaseAmount = 197.55;
double tax = purchaseAmount * 0.06;
System.out.println("Sales tax is $" + tax);
System.out.println("Sales tax is $" +
(int)(tax * 100) / 100.0);
}
}
Sales tax is $11.853
Sales tax is $11.85

37
Character Data Type
char letter = 'A'; (ASCII)
char numChar = '4'; (ASCII)
char letter = 'u0041'; (Unicode)
char numChar = 'u0034'; (Unicode)
Four hexadecimal digits.
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);

38
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

39
Problem: Displaying Unicodes
Write a program that displays two Chinese
characters and three Greek letters.
import javax.swing.JOptionPane;
public class DisplayUnicode {
JOptionPane.showMessageDialog(null,
"u6B22u8FCE u03b1 u03b2 u03b3",
"u6B22u8FCE Welcome",
JOptionPane.INFORMATION_MESSAGE);
}
}

40
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

41
Appendix B: ASCII Character Set
ASCII Character Set is a subset of the Unicode from u0000 to u007f
1
2

42
ASCII Character Set, cont.
ASCII Character Set is a subset of the Unicode from u0000 to u007f
1
2

43
Casting between char and Numeric Types
int i = 'a'; // Same as int i = (int)'a';
char c = 97; // Same as char c = (char)97;

44
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.

45
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'; // s becomes SupplementB
Create a String type named message and
store “Welcome to Java” at that location
Create a String type named s and store
“Chapter2” at that location
Create a String type named s1 and store
“SupplementB” at that location

46
String Concatenation (cont.)
// add to message
String message = "Welcome " + "to " + "Java";
Message = message + “! Good luck!!”
Create a String type named message and store “Welcome to Java” at that
location. Then add “! Good luck!!” to message and now “Welcome to Jave! Good
luck!!” is stored at location message.

47
Obtaining Input
This book provides two ways of obtaining input.
1. Using JOptionPane input dialogs (§2.11)
Inputs from a GUI window.
2. Using the JDK 1.5 Scanner class (§2.16)
Inputs from DOS window or lower pane in
Eclipse workspace.

48
Getting Input Using Scanner
1. Need to import Scanner
import java.util.Scanner;
2. Create a Scanner object
Scanner scanner = new Scanner(System.in);
3. 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 scanner = new Scanner(System.in);

49
TestScanner
 TestScanner.java
import java.util.Scanner; // Scanner is in java.util
public class TestScanner {
public static void main(String args[]) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter an integer
System.out.print("Enter an integer: ");
int intValue = input.nextInt();
System.out.println("You entered the integer " + intValue);
// Prompt the user to enter a double value
System.out.print("Enter a double value: ");
double doubleValue = input.nextDouble();
System.out.println("You entered the double value " + doubleValue);
// Prompt the user to enter a string
System.out.print("Enter a string without space: ");
String string = input.next();
System.out.println("You entered the string " + string);
}
}

50
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.
12
)1(
11 ×
+
−
×
arsnumberOfYe
erestRatemonthlyInt
erestRatemonthlyIntloanAmount

51
ComputeLoan.java
public class ComputeLoan {
// Create a Scanner
// Enter yearly interest rate
System.out.print("Enter yearly interest rate, for example 8.25: ");
double annualInterestRate = input.nextDouble();
// Obtain monthly interest rate
double monthlyInterestRate = annualInterestRate / 1200;
// Enter number of years
System.out.print( "Enter number of years as an integer, for example 5: ");
int numberOfYears = input.nextInt();
// Enter loan amount
System.out.print("Enter loan amount, for example 120000.95: ");
double loanAmount = input.nextDouble();
// Calculate payment
double monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1 / Math.pow(1 + monthlyInterestRate,
numberOfYears * 12));
double totalPayment = monthlyPayment * numberOfYears * 12;
// Format to keep two digits after the decimal point
monthlyPayment = (int)(monthlyPayment * 100) / 100.0;
totalPayment = (int)(totalPayment * 100) / 100.0;
// Display results System.out.println("The monthly payment is " + monthlyPayment);
System.out.println("The total payment is " + totalPayment);
}
}

52
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.

53
ComputeChange.java
public class ComputeChange {
// Create a Scanner
// Receive the amount
System.out.print( "Enter an amount in double, for example 11.56: ");
double amount = input.nextDouble();
int remainingAmount = (int)(amount * 100);
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100; remainingAmount = remainingAmount % 100;
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25; remainingAmount = remainingAmount % 25;
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10; remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
// Display results
String output = "Your amount " + amount + " consists of n" + "t" + numberOfOneDollars + " dollarsn" + "t" +
numberOfQuarters + " quartersn" + "t" + numberOfDimes + " dimesn" + "t" + numberOfNickels + " nickelsn" + "t" +
numberOfPennies + " pennies"; System.out.println(output);
}
}

54
Trace ComputeChange
int numberOfOneDollars = remainingAmount / 100;
int numberOfQuarters = remainingAmount / 25;
int numberOfDimes = remainingAmount / 10;
1156remainingAmount
remainingAmount
initialized
Suppose amount is 11.56

55
Trace ComputeChange
1156remainingAmount
11numberOfOneDollars
numberOfOneDollars
assigned
animation

56
Trace ComputeChange
56remainingAmount
remainingAmount
updated
animation

57
Trace ComputeChange
56remainingAmount
2numberOfOneQuarters
numberOfOneQuarters
assigned
animation

58
Trace ComputeChange
6remainingAmount
2numberOfQuarters
remainingAmount
updated
animation

59
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.
Elapsed
time
Unix Epoch
01-01-1970
00:00:00 GMT
Current Time
Time
System.CurrentTimeMills()

60
ShowCurrentTime.java
public class ShowCurrentTime {
// Obtain the total milliseconds since the midnight, Jan 1, 1970
long totalMilliseconds = System.currentTimeMillis();
// Obtain the total seconds since the midnight, Jan 1, 1970
long totalSeconds = totalMilliseconds / 1000;
// Compute the current second in the minute in the hour
int currentSecond = (int)(totalSeconds % 60);
// Obtain the total minutes
long totalMinutes = totalSeconds / 60;
// Compute the current minute in the hour
int currentMinute = (int)(totalMinutes % 60);
// Obtain the total hours
long totalHours = totalMinutes / 60;
// Compute the current hour
int currentHour = (int)(totalHours % 24);
// Display results
System.out.println("Current time is " + currentHour + ":" + currentMinute + ":" +
currentSecond + " GMT");
}
}

61
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:
import javax.swing.JOptionPane; //need to import the method
//the import goes before public class.
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.

62
Getting Input from Input Dialog Boxes
String input = JOptionPane.showInputDialog(
"Enter an input");

63
Getting Input from Input Dialog Boxes
String string = JOptionPane.showInputDialog(
null, “Prompting Message”, “Dialog Title”,
JOptionPane.QUESTION_MESSAGE);

64
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:
String intString = JOptionPane.showInputDialog(null, “Enter
an integer”, “Integer Example”,
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as “123”.

65
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:
String doubleString =
JOptionPane.showInputDialog(null, “Enter a double
value”, “Double Example”,
double doubleValue =Double.parseDouble(doubleString);
where doubleString is a numeric string such as “123.45”.

66
Dialog Box Input Conversion
 Input from JOptionPane.showInputDialog is
a String type
 Need to convert the input into type required
 byte byteValue = Byte.parseByte(intString);
 int intValue = Integer.parseInt(intString);
 long longValue = Long.parserLong(intString);
 float floatValue = Float.parseFloat(intString);
 double doubleValue
=Double.parseDouble(doubleString);
 Since the input value is a String, don’t need to
convert the input into a String type

67
Problem: Computing Loan Payments Using Input Dialogs
ComputeLoanUsingInputDialogComputeLoanUsingInputDialog RunRun
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.
12
)1(
11 ×
+
−
×
arsnumberOfYe
erestRatemonthlyInt
erestRatemonthlyIntloanAmount

68
ComputeLoanUsingInputDialog
import javax.swing.JOptionPane;
public class ComputeLoanUsingInputDialog {
// Enter yearly interest rate
String annualInterestRateString = JOptionPane.showInputDialog( "Enter yearly interest rate, for example 8.25:");
// Convert string to double
double annualInterestRate = Double.parseDouble(annualInterestRateString);
// Obtain monthly interest rate
double monthlyInterestRate = annualInterestRate / 1200;
// Enter number of years
String numberOfYearsString = JOptionPane.showInputDialog( "Enter number of years as an integer, nfor example
5:");
// Convert string to int
int numberOfYears = Integer.parseInt(numberOfYearsString);
// Enter loan amount
String loanString = JOptionPane.showInputDialog( "Enter loan amount, for example 120000.95:");
// Convert string to double
double loanAmount = Double.parseDouble(loanString);
// Calculate payment
double monthlyPayment = loanAmount * monthlyInterestRate / (1 - 1 / Math.pow(1 + monthlyInterestRate,
numberOfYears * 12));
double totalPayment = monthlyPayment * numberOfYears * 12;
// Format to keep two digits after the decimal point
monthlyPayment = (int)(monthlyPayment * 100) / 100.0;
totalPayment = (int)(totalPayment * 100) / 100.0;
// Display results
String output = "The monthly payment is " + monthlyPayment + "nThe total payment is " + totalPayment;
JOptionPane.showMessageDialog(null, output);
}
}

69
Programming Style and Documentation
 Appropriate Comments
 Naming Conventions
 Proper Indentation and Spacing Lines
 Block Styles

70
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, due date, date submitted, program
name and a brief description at the beginning of the
program.
/****************************************
*
* Student Name:
* Date Due:
* Date Submitted:
* Program Name:
* Program Description:
*
****************************************/

71
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.

72
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

73
Proper Indentation and Spacing
 Indentation
 Follow Eclipse indention.
 Spacing
 Use blank line to separate segments of the
code.

74
Block Styles
Use end-of-line style for braces.
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
Next-line
style

75
Programming Errors
 Syntax Errors
 Detected by the compiler
 Runtime Errors
 Causes the program to abort
 Logic Errors
 Produces incorrect result

76
Syntax Errors
public class ShowSyntaxErrors {
i = 30;
System.out.println(i + 4);
}
}
Did not declare variable type for i

77
Runtime Errors
public class ShowRuntimeErrors {
int i = 1 / 0;
}
}
Cannot divide by 0

78
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);
}
}
Does not print correct results for number2

79
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.

80
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.

Ch3

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