CHAPTER 10

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CHAPTER 10 EVENT HANDLING

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CHAPTER GOALS

• To understand the Java event model

• To install mouse and action listeners

• To accept mouse and text input

• To display frame windows

• To show text output in a text area with a

scroll bar

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Event Classes

• Event source: Generates events and manages listeners Example: applet

• Listener: Notified when event happens Example: MouseListener

• Event class: Describes event Example: ^MouseEvent

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The

MouseListener

Interface

public interface MouseListener {

void mousePressed(MouseEvent event);

//Called when a mouse button has been pressed on a component

void mouseReleased(MouseEvent event);

//Called when a mouse button has been released on a component

void mouseClicked(MouseEvent event);

//Called when the mouse has been clicked on a component void mouseEntered(MouseEvent event);

//Called when the mouse enters a component void mouseExited(MouseEvent event);

//Called when the mouse exits a component }

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File MouseSpy.java

1 import java.awt.event.MouseEvent;

2 import java.awt.event.MouseListener;

3 4 /**

5 This listener simply prints out the listener method name 6 and the x- and y-coordinate of the mouse position.

7 */

8 public class MouseSpy implements MouseListener 9 {

10 public void mousePressed(MouseEvent event) 11 {

12 System.out.println("Mouse pressed. x = "

13 + event.getX() + " y = " + event.getY());

14 } 15

16 public void mouseReleased(MouseEvent event) 17 {

18 System.out.println("Mouse released. x = "

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20 } 21

22 public void mouseClicked(MouseEvent event) 23 {

24 System.out.println("Mouse clicked. x = "

26 } 27

28 public void mouseEntered(MouseEvent event) 29 {

30 System.out.println("Mouse entered. x = "

32 } 33

34 public void mouseExited(MouseEvent event) 35 {

36 System.out.println("Mouse exited. x = "

38 } 39 }

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File MouseSpyApplet.java

1 import java.applet.Applet;

2 3 /**

4 This applet installs a mouse spy. Try generating the

5 five mouse event types by moving and clicking the mouse.

6 */

7 public class MouseSpyApplet extends Applet 8 {

9 public MouseSpyApplet() 10 {

11 MouseSpy listener = new MouseSpy();

12 addMouseListener(listener);

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Spying on Mouse Events

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Mouse Listener that Moves a Rectangle

public MouseApplet() {

...

//add mouse press listener

class MousePressListener implements MouseListener {

public void mousePressed(MouseEvent event) {

int x = event.getX();

int y = event.getY();

box.setLocation(x,y);

repaint();

}

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//do-nothing methods

public void mouseReleased(MouseEvent event) {} public void mouseClicked(MouseEvent event){}

public void mouseEntered(MouseEvent event){}

public void mouseExited(MouseEvent event){}

} MouseListener listener = new MousePressListener();

addMouseListener(listener);

}

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Mouse Listener that Moves a Rectangle

• Gets mouse position from event object

• Accesses box instance field of outer class

• Invokes repaint method on outer class

• repaint triggers paint method

• You should always call repaint to trigger painting Don't call ^paint directly

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The Mouse Applet

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The Mouse Applet

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An Applet with a Control Panel

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Control Panel Components

• Use Swing package javax.swing

• Class names start with J

• JTextField xField = new JTextField(width);

• JLabel xLabel = new JLabel("x = ");

• Icon buttonIcon = new ImageIcon("hand.gif");

• moveButton = new JButton("Move",buttonIcon);

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Button Listener

• Moves rectangle when button is clicked

• Reads xField, yField variables from enclosing scope public ButtonApplet()

{ ...

class MoveButtonListener implements ActionListener {

public void actionPerformed(ActionEvent event) {

int x =Integer.parseInt(xField.getText());

int y =Integer.parseInt(yField.getText());

box.setLocation(x,y);

repaint();

} };

ActionListener listener = new MoveButtonListener();

moveButton.addActionListener(listener);

...

}

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Placing Components in a Panel

• JPanel is container for components

• Add components to panel

JPanel panel = new JPanel();

panel.add(xLabel);

panel.add(xField);

. . .

• Place panel inside frame

JFrame frame = new JFrame();

frame.setContentPane(panel);

• Pack frame: size it to just contain the components frame.pack();

frame.show();

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File ButtonApplet.java

2 import java.awt.Graphics;

3 import java.awt.Graphics2D;

4 import java.awt.Rectangle;

5 import java.awt.event.ActionEvent;

6 import java.awt.event.ActionListener;

7 import javax.swing.ImageIcon;

8 import javax.swing.JButton;

9 import javax.swing.JFrame;

10 import javax.swing.JLabel;

11 import javax.swing.JPanel;

12 import javax.swing.JTextField;

13 14 /**

15 This applet lets the user move a rectangle by specifying 16 the x- and y-position of the top left corner.

17 */

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18 public class ButtonApplet extends Applet 19 {

20 public ButtonApplet() 21 {

22 // the rectangle that the paint method draws 23 box = new Rectangle(BOX_X, BOX_Y, 24 BOX_WIDTH, BOX_HEIGHT);

25

26 // the text fields for entering the x- and y-coordinates 27 final JTextField xField = new JTextField(5);

28 final JTextField yField = new JTextField(5);;

29

30 // the button to move the rectangle

31 JButton moveButton = new JButton("Move", 32 new ImageIcon("hand.gif"));

33

34 class MoveButtonListener implements ActionListener 35 {

36 public void actionPerformed(ActionEvent event) 37 {

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38 int x = Integer.parseInt(xField.getText());

39 int y = Integer.parseInt(yField.getText());

40 box.setLocation(x, y);

41 repaint();

42 } 43 };

44

45 ActionListener listener = new MoveButtonListener();

46 moveButton.addActionListener(listener);

47

48 // the labels for labeling the text fields 49 JLabel xLabel = new JLabel("x = ");

50 JLabel yLabel = new JLabel("y = ");

51

52 // the panel for holding the user interface components 53 JPanel panel = new JPanel();

54

55 panel.add(xLabel);

56 panel.add(xField);

57 panel.add(yLabel);

58 panel.add(yField);

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59 panel.add(moveButton);

60

61 // the frame for holding the component panel 62 JFrame frame = new JFrame();

63 frame.setContentPane(panel);

64 frame.pack();

65 frame.show();

66 } 67

68 public void paint(Graphics g) 69 {

70 Graphics2D g2 = (Graphics2D)g;

71 g2.draw(box);

72 } 73 74 private Rectangle box;

75 private static final int BOX_X = 100;

76 private static final int BOX_Y = 100;

77 private static final int BOX_WIDTH = 20;

78 private static final int BOX_HEIGHT = 30;

79 }

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Multiple Buttons with Similar Behavior

• Don't want to write separate code for similar listeners

• Remedy: Write method that attaches the listener

• Make variable parts into ^final method parameters

• Local listener class can access ^final parameters

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An Applet with Multiple Buttons

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Method for Attaching Listener

public JButton makeButton(String label, final int dx,final int dy) {

JButton button = new JButton(label);

class ButtonListener implements ActionListener {

public void actionPerformed(ActionEvent event) {

box.translate(dx, dy);

repaint();

} };

ButtonListener listener = new ButtonListener();

button.addActionListener(listener);

return button;

}

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Method for Attaching Listener

• Call multiple times

panel.add(makeButton("Left",-BOX_WIDTH,0));

panel.add(makeButton("Right",BOX_WIDTH,0));

panel.add(makeButton("Up",0,-BOX_HEIGHT));

panel.add(makeButton("Down",0,BOX_HEIGHT));

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File ButtonApplet.java

2 import java.awt.Graphics;

3 import java.awt.Graphics2D;

4 import java.awt.Rectangle;

10 11 /**

12 This applet lets the user move a rectangle by clicking 13 on buttons labeled "Left", "Right", "Up", and "Down".

14 */

15 public class ButtonApplet extends Applet 16 {

17 public ButtonApplet()

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18 {

19 // the rectangle that the paint method draws 20 box = new Rectangle(BOX_X, BOX_Y, 21 BOX_WIDTH, BOX_HEIGHT);

22

23 // the panel for holding the user interface components 24 JPanel panel = new JPanel();

25

26 panel.add(makeButton("Left", -BOX_WIDTH, 0));

27 panel.add(makeButton("Right", BOX_WIDTH, 0));

28 panel.add(makeButton("Up", 0, -BOX_HEIGHT));

29 panel.add(makeButton("Down", 0, BOX_HEIGHT));

30

31 // the frame for holding the component panel 32 JFrame frame = new JFrame();

34 frame.pack();

35 frame.show();

36 } 37

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38 public void paint(Graphics g) 39 {

40 Graphics2D g2 = (Graphics2D)g;

41 g2.draw(box);

42 } 43 44 /**

45 Makes a button that moves the box.

46 @param label the label to show on the button

47 @param dx the amount by which to move the box in x-direction 48 when the button is clicked

49 @param dy the amount by which to move the box in y-direction 50 when the button is clicked

51 @return the button 52 */

53 public JButton makeButton(String label, final int dx, 54 final int dy)

55 {

56 JButton button = new JButton(label);

57

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58 class ButtonListener implements ActionListener 59 {

62 box.translate(dx, dy);

63 repaint();

64 } 65 };

66

67 ButtonListener listener = new ButtonListener();

68 button.addActionListener(listener);

69 return button;

70 } 71 72 private Rectangle box;

73 private static final int BOX_X = 100;

74 private static final int BOX_Y = 100;

75 private static final int BOX_WIDTH = 20;

76 private static final int BOX_HEIGHT = 30;

77 }

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Frame Windows

• Application program, not applet

• Construct and show frame

JFrame frame = new JFrame();

. . .

frame.show();

• Set the default close operation

frame.setDefaultCloseOperation(EXIT_ON_CLOSE);

• Add components to a panel, then set the panel as content pane

JPanel panel = new JPanel();

panel.add(. . .);

panel.add(. . .)

frame.setContentPane(panel);

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A Frame with Two Labels

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File FrameTest.java

1 import javax.swing.ImageIcon;

5 6 /**

7 This program displays a frame with an image and a text label.

8 */

9 public class FrameTest 10 {

11 public static void main(String[] args) 12 {

13 JFrame frame = new JFrame();

14 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

15

16 JLabel iconLabel = new JLabel(new ImageIcon("world.gif"));

17 JLabel textLabel = new JLabel("Hello, World!");

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18

19 JPanel panel = new JPanel();

20 panel.add(iconLabel);

21 panel.add(textLabel);

23

24 frame.pack();

25 frame.show();

26 } 27 }

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Text Components

• JTextField holds a single line of text

• JTextArea holds multiple lines

• Construct with new JTextArea(rows, columns)

• textArea.append(aString) appends text

• Use textArea.setEditable(false) to use for display only

• To add scroll bars, use

JScrollPane scrollPane = new JScrollPane(textArea);

textArea.setContentPane(scrollPane);

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A Text Area with Scroll Bars

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The Control Panel for Adding

Interest

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File TextAreaTest.java

7 import javax.swing.JScrollPane;

8 import javax.swing.JTextArea;

9 import javax.swing.JTextField;

10 11 /**

12 This program shows a frame with a text area that displays 13 the growth of an investment. A second frame holds a text 14 field to specify the interest rate.

15 */

16 public class TextAreaTest 17 {

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18 public static void main(String[] args) 19 {

20 // the application adds interest to this bank account

21 final BankAccount account = new BankAccount(INITIAL_BALANCE);

22 // the text area for displaying the results

23 final JTextArea textArea = new JTextArea(10, 30);

24 textArea.setEditable(false);

25 JScrollPane scrollPane = new JScrollPane(textArea);

26

27 // construct the frame for displaying the text area 28 JFrame frame = new JFrame();

29 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

30 frame.setContentPane(scrollPane);

31 frame.pack();

32 frame.show();

33

34 // the label and text field for entering the interest rate 35 JLabel rateLabel = new JLabel("Interest Rate: ");

36

37 final JTextField rateField = new JTextField(10);

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38 rateField.setText("" + DEFAULT_RATE);

39

40 // the button to trigger the calculation

41 JButton calculateButton = new JButton("Add Interest");

42

43 class CalculateListener implements ActionListener 44 {

47 double rate = Double.parseDouble(

48 rateField.getText());

49 double interest = account.getBalance() 50 * rate / 100;

51 account.deposit(interest);

52 textArea.append(account.getBalance() + "\n");

53 } 54 }

55

56 ActionListener listener = new CalculateListener();

57 calculateButton.addActionListener(listener);

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58

59 // the control panel that holds the input components 60 JPanel controlPanel = new JPanel();

61 controlPanel.add(rateLabel);

62 controlPanel.add(rateField);

63 controlPanel.add(calculateButton);

64

65 // the frame to hold the control panel 66 JFrame controlFrame = new JFrame();

67 controlFrame.setContentPane(controlPanel);

68 controlFrame.pack();

69 controlFrame.show();

70 } 71

72 private static final double DEFAULT_RATE = 10;

73 private static final double INITIAL_BALANCE = 1000;

74 }