EVENT HANDLING

1
CHAPTER 10

• EVENT HANDLING

2
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

3
Event Classes

• Event source Generates events and manages
  listenersExample applet
• Listener Notified when event happensExample
  MouseListener
• Event class Describes eventExample MouseEvent

4
The MouseListener Interface

• public interface MouseListener
• void mousePressed(MouseEvent event)
  //Called when a mouse button has been pressed on
  a componentvoid mouseReleased(MouseEvent
  event)  //Called when a mouse button has been
  released on a componentvoid mouseClicked(MouseEv
  ent event) //Called when the mouse has been
  clicked on a componentvoid
  mouseEntered(MouseEvent event) //Called when
  the mouse enters a component void
  mouseExited(MouseEvent event) //Called when
  the mouse exits a component

5
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
  "

6

• 19 event.getX() " y "
  event.getY())
• 20
• 21
• 22 public void mouseClicked(MouseEvent event)
• 23
• 24 System.out.println("Mouse clicked. x "
  
• 25 event.getX() " y "
  event.getY())
• 26
• 27
• 28 public void mouseEntered(MouseEvent event)
• 29
• 30 System.out.println("Mouse entered. x "
  
• 31 event.getX() " y "
  event.getY())
• 32
• 33
• 34 public void mouseExited(MouseEvent event)
• 35
• 36 System.out.println("Mouse exited. x "
  
• 37 event.getX() " y "
  event.getY())

7
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)
• 13
• 14
• 15

8
Spying on Mouse Events
9
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()

10

• //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)

11
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

12
The Mouse Applet
13
The Mouse Applet
14
An Applet with a Control Panel
15
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)

16
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(xFie
  ld.getText()) int y Integer.parseInt(yFi
  eld.getText()) box.setLocation(x,y)
  repaint() ActionListener
  listener new MoveButtonListener()
  moveButton.addActionListener(listener) ...

17
Placing Components in a Panel

• JPanel is container for components
• Add components to panelJPanel panel new
  JPanel()panel.add(xLabel)panel.add(xField).
  . .
• Place panel inside frameJFrame frame new
  JFrame()frame.setContentPane(panel)
• Pack frame size it to just contain the
  componentsframe.pack()frame.show()

18
File ButtonApplet.java

• 1 import java.applet.Applet
• 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 /

19

• 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(ActionEven
  t event)

20

• 38 int x Integer.parseInt(xField.get
  Text())
• 39 int y Integer.parseInt(yField.get
  Text())
• 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)

21

• 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

22
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

23
An Applet with Multiple Buttons
24
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(list
  ener) return button

25
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))

26
File ButtonApplet.java

• 1 import java.applet.Applet
• 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.JButton
• 8 import javax.swing.JFrame
• 9 import javax.swing.JPanel
• 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()

27

• 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()
• 33 frame.setContentPane(panel)
• 34 frame.pack()
• 35 frame.show()
• 36

28

• 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 _at_param label the label to show on the
  button
• 47 _at_param dx the amount by which to move
  the box in x-direction
• 48 when the button is clicked
• 49 _at_param dy the amount by which to move
  the box in y-direction
• 50 when the button is clicked
• 51 _at_return the button
• 52 /
• 53 public JButton makeButton(String label,
  final int dx,
• 54 final int dy)
• 55
• 56 JButton button new JButton(label)

29

• 58 class ButtonListener implements
  ActionListener
• 59
• 60 public void actionPerformed(ActionEven
  t event)
• 61
• 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

30
Frame Windows

• Application program, not applet
• Construct and show frameJFrame frame new
  JFrame(). . .frame.show()
• Set the default close operationframe.setDefaultCl
  oseOperation(EXIT_ON_CLOSE)
• Add components to a panel, then set the panel as
  content paneJPanel panel new
  JPanel()panel.add(. . .)panel.add(. .
  .)frame.setContentPane(panel)

31
A Frame with Two Labels
32
File FrameTest.java

• 1 import javax.swing.ImageIcon
• 2 import javax.swing.JFrame
• 3 import javax.swing.JLabel
• 4 import javax.swing.JPanel
• 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.EXI
  T_ON_CLOSE)
• 15
• 16 JLabel iconLabel new JLabel(new
  ImageIcon("world.gif"))
• 17 JLabel textLabel new JLabel("Hello,
  World!")

33

• 18
• 19 JPanel panel new JPanel()
• 20 panel.add(iconLabel)
• 21 panel.add(textLabel)
• 22 frame.setContentPane(panel)
• 23
• 24 frame.pack()
• 25 frame.show()
• 26
• 27

34
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, useJScrollPane scrollPane
  new JScrollPane(textArea)textArea.setContentPane
  (scrollPane)

35
A Text Area with Scroll Bars

36
The Control Panel for Adding Interest
37
File TextAreaTest.java

• 1 import java.awt.event.ActionEvent
• 2 import java.awt.event.ActionListener
• 3 import javax.swing.JButton
• 4 import javax.swing.JFrame
• 5 import javax.swing.JLabel
• 6 import javax.swing.JPanel
• 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

38

• 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.EXI
  T_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

39

• 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
• 45 public void actionPerformed(ActionEven
  t event)
• 46
• 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()

40

• 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