Tutorial 9 – Class Average Application

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Outline

9.1 Test Driving the Class Average Application 9.2 do…while Repetition Statement

9.3 Creating the Class Average Application 9.4 Wrap-Up

Tutorial 9 – Class Average Application

Introducing the do … while Repetition Statement

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Objectives

• In this tutorial, you will learn to:

– Use the

do…while

statement.

– Understand counter-controlled repetition.

– Display an input dialog.

– Enable and disable

JButton

s.

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9.1 Test Driving the Class Average Application

Application Requirements

A teacher gives quizzes to a class of 10 students. The grades on these quizzes are integers in the range from 0 to 100, inclusive (0 and 100 are each valid grades). The teacher would like you to develop an application that computes the class average for a quiz. Your application should use an input dialog to enable the teacher to enter the grades.

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Application (Cont.)

Figure 9.1 Running the completed Class Average application.

JTextArea Disabled JButton

Output JTextField

• Display input dialog

– Click Get Grades JButton

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9.1 Test Driving the Class Average Application (Cont.)

Figure 9.2 Entering grades in the Class Average application.

JTextField of input dialog

• Entering grades

– Enter 85 as the first grade – Click the OK JButton

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Application (Cont.)

Figure 9.3 Class Average application after 10 grades have been entered.

Ten quiz grades entered

• Entering more grades

– Complete entering the students’ grades

• Focus is set on Average JButton

Enabled JButton

Blue rectangle indicates that this component has the focus

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9.1 Test Driving the Class Average Application (Cont.)

Figure 9.4 Class Average application after calculating the average.

• Calculating average

– Press the Space Bar to calculate average

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9.2 do … while Repetition Statement

• do … while Repetition statement

– Similar to the

while

statement

– Iterates while loop-continuation condition is

true

– Loop-continuation condition checked after the body of the loop is performed

– Body always executes at least once

• Off-by-one errors

– Occur when the loop executes one too many or one less

iteration than is necessary

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9.2 do … while Repetition Statement (Cont.)

Figure 9.5 do…while repetition statement UML activity diagram.

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9.3 Creating the Class Average Application

Retrieve grades from user when the user clicks the Get Grades JButton:

Set total to zero

Set grade counter to one Clear the JTextArea

Clear the output JTextField Do

Get the next grade from the input dialog Append the grade to the JTextArea Add the grade to the total

Add one to the grade counter

While the grade counter is less than or equal to 10

Enable Average JButton

Give focus to Average JButton

Calculate average when the user clicks the Average JButton:

Calculate the class average by dividing the total by 10 Display the class average in the output JTextField Disable Average JButton

Give focus to Get Grades JButton

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9.3 Creating the Class Average Application (Cont.)

Action Component Event

Label the application’s components

gradeListJLabel, classAverageJLabel

Retrieve grades from user getGradesJButton User clicks Get Grades JButton

Clear the JTextArea gradeListJTextArea

Clear the output JTextField classAverageJTextField

Get the next grade from the input dialog

JOptionPane

Append the grade to the JTextArea

gradeListJTextArea

Enable Average JButton averageJButton

Give focus to Average JButton

averageJButton

Calculate average averageJButton User clicks Average JButton

Display the class average classAverageJTextField

Disable Average JButton averageJButton

Give focus to Get Grades JButton

getGradesJButton

Figure 9.6 ACE table for the Class Average application.

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(Cont.)

• Local variables

– Can only be used in the body of the method in which they are declared

• Instance variables

– Defined within a class, but outside any methods

– Can be accessed from any method

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9.3 Creating the Class Average Application (Cont.)

Figure 9.7 Variable total declared outside a method.

Figure 9.8 Initializing your application’s variables.

Declaring local variables

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(Cont.)

• Retrieving data from an input dialog

– Call the

JOptionPane.showInputDialog

method to display an input dialog

– Returns the input entered by the user as a

String

when the user clicks the

^OK JButton

– Use

Integer.parseInt

to convert the

String

to an

int

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9.3 Creating the Class Average Application (Cont.)

Figure 9.9 Clearing the output components.

Figure 9.10 Getting the grade input using an input dialog.

Clearing the grade list and class average

Retrieving data from an input dialog

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(Cont.)

Figure 9.11 Adding the grade input to the gradeListJTextArea.

Appending the input value to the gradeListJTextArea

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9.3 Creating the Class Average Application (Cont.)

Figure 9.12 Running the updated application.

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(Cont.)

Figure 9.13 Defining the do…while loop.

Start of do…while statement

Incrementing ^counter for next iteration Condition of do…

while statement

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9.3 Creating the Class Average Application (Cont.)

Figure 9.14 Running the updated application.

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(Cont.)

Figure 9.15 Summing the grades.

Figure 9.16 Disabling a JButton.

Add input to total

Use method setEnabled to enable or disable a ^JButton

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9.3 Creating the Class Average Application (Cont.)

• setEnabled method

– Pass the argument

true

to allow the user to press the

JButton

(enable the

JButton

)

– Pass the argument

false

to prevent the user from pressing

the

JButton

(disable the

JButton

)

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(Cont.)

Figure 9.17 Enabling a JButton.

Figure 9.18 Transferring focus to a JButton.

To enable a JButton or other GUI component, pass ^true to method setEnabled

Transfer focus to a GUI component by using method

requestFocusInWindow

• Transferring the focus

– Call the JButton’s requestFocusInWindow method

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9.3 Creating the Class Average Application (Cont.)

Figure 9.19 Calculating and displaying the class average.

Calculating and displaying the class average

• Cast operator

– Convert the operand (in this case total) to the type placed within the parentheses of the cast.

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(Cont.)

Figure 9.20 Completed Class Average application.

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Outline

ClassAverage.java (1 of 7)

2 // Application enables user to have the average of grades calculated.

3 import java.awt.*;

4 import java.awt.event.*;

5 import javax.swing.*;

6

7 public class ClassAverage extends JFrame 8 {

9 // JLabel and JTextArea for list of grades 10 private JLabel gradeListJLabel;

11 private JTextArea gradeListJTextArea;

12

13 // JButton initiates retrieving grades 14 private JButton getGradesJButton;

15

16 // JButton initiates calculating average 17 private JButton averageJButton;

18

19 // JLabel and JTextField used to display average 20 private JLabel classAverageJLabel;

21 private JTextField classAverageJTextField;

22

23 private int total = 0; // holds value of the grade total

24 Declaring an

instance variable

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27 {

28 createUserInterface();

29 } 30

31 // create and position GUI components; register event handlers 32 private void createUserInterface()

33 {

34 // get content pane for attaching GUI components 35 Container contentPane = getContentPane();

36

37 // enable explicit positioning of GUI components 38 contentPane.setLayout( null );

39

40 // set up gradeListJLabel

41 gradeListJLabel = new JLabel();

42 gradeListJLabel.setBounds( 16, 8, 70, 23 );

43 gradeListJLabel.setText( "Grade list:" );

44 contentPane.add( gradeListJLabel );

45

46 // set up gradeListJTextArea

47 gradeListJTextArea = new JTextArea();

48 gradeListJTextArea.setBounds( 16, 32, 88, 180 );

49 contentPane.add( gradeListJTextArea );

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Outline

51 // set up getGradesJButton

52 getGradesJButton = new JButton();

53 getGradesJButton.setBounds( 128, 50, 100, 26 );

54 getGradesJButton.setText( "Get Grades" );

55 contentPane.add( getGradesJButton );

56 getGradesJButton.addActionListener(

57

58 new ActionListener() // anonymous inner class 59 {

60 // event handler called when getGradesJButton is clicked 61 public void actionPerformed( ActionEvent event )

62 {

63 getGradesJButtonActionPerformed( event );

64 } 65

66 } // end anonymous inner class 67

68 ); // end call to addActionListener 69

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72 averageJButton.setBounds( 128, 90, 100, 26 );

73 averageJButton.setText( "Average" );

74 averageJButton.setEnabled( false );

75 contentPane.add( averageJButton );

76 averageJButton.addActionListener(

77

78 new ActionListener() // anonymous inner class 79 {

80 // event handler called when averageJButton is clicked 81 public void actionPerformed( ActionEvent event )

82 {

83 averageJButtonActionPerformed( event );

84 } 85

86 } // end anonymous inner class 87

88 ); // end call to addActionListener 89

90 // set up classAverageJLabel

91 classAverageJLabel = new JLabel();

92 classAverageJLabel.setBounds( 128, 132, 90, 23 );

93 classAverageJLabel.setText( "Class average:" );

94 contentPane.add( classAverageJLabel );

Disabling the

Average JButton

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Outline

96 // set up classAverageJTextField

97 classAverageJTextField = new JTextField();

98 classAverageJTextField.setBounds( 128, 156, 100, 21 );

99 classAverageJTextField.setEditable( false );

100 classAverageJTextField.setHorizontalAlignment(

101 JTextField.CENTER );

102 contentPane.add( classAverageJTextField );

103

104 // set properties of application’s window

105 setTitle( "Class Average" ); // set title bar text 106 setSize( 250, 250 ); // set window size 107 setVisible( true ); // display window 108

109 } // end method createUserInterface 110

111 // method retrieves, totals, and displays grades from user

112 private void getGradesJButtonActionPerformed( ActionEvent event ) 113 {

114 total = 0; // stores total of grades entered 115 int counter = 1; // counter controls do...while statement 116 String input; // stores data entered into input dialog 117 int grade; // stores int value converted from input 118

Declaring local variables

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121 classAverageJTextField.setText( "" );

122

123 do 124 { 125 // get user input 126 input = JOptionPane.showInputDialog( null, "Enter Grade" );

127 grade = Integer.parseInt( input );

128

129 // add text to output 130 gradeListJTextArea.append( grade + "\n" );

131 total += grade; // add input to total 132 counter++; // increment counter 133 } 134 while ( counter <= 10 ); // end do...while 135

136 averageJButton.setEnabled( true ); // enable averageJButton 137 averageJButton.requestFocusInWindow(); // transfer focus 138

139 } // end method getGradesJButtonActionPerformed 140

Clearing a JTextArea

and ^JTextField

Enabling the

Average JButton

and giving it the focus

Using a do…while

statement to calculate the class average

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Outline

142 private void averageJButtonActionPerformed( ActionEvent event ) 143 {

144 double average = ( double ) total / 10; // calculate average 145 classAverageJTextField.setText( String.valueOf( average ) );

146

147 } // end method averageJButtonActionPerformed 148

149 // main method

150 public static void main( String[] args ) 151 {

152 ClassAverage application = new ClassAverage();

153 application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

154

155 } // end method main 156

157 } // end class ClassAverage

Calculating and displaying class average