CHAPTER 10 EVENT HANDLING

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

Event Classes Event source: Generates events and manages listeners Example: applet Listener: Notified when event happens Example: MouseListener Event class: Describes event Example: MouseEvent

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 }

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 } public void mouseReleased(MouseEvent event) 17 { 18 System.out.println("Mouse released. x = "

19 + event.getX() + " y = " + event.getY()); 20 } public void mouseClicked(MouseEvent event) 23 { 24 System.out.println("Mouse clicked. x = " 25 + event.getX() + " y = " + event.getY()); 26 } public void mouseEntered(MouseEvent event) 29 { 30 System.out.println("Mouse entered. x = " 31 + event.getX() + " y = " + event.getY()); 32 } public void mouseExited(MouseEvent event) 35 { 36 System.out.println("Mouse exited. x = " 37 + event.getX() + " y = " + event.getY()); 38 } 39 }

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

Spying on Mouse Events

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(); }

//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); }

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

The Mouse Applet

An Applet with a Control Panel

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);

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);... }

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();

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; /** 15 This applet lets the user move a rectangle by specifying 16 the x- and y-position of the top left corner. 17 */

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); // the text fields for entering the x- and y-coordinates 27 final JTextField xField = new JTextField(5); 28 final JTextField yField = new JTextField(5);; // the button to move the rectangle 31 JButton moveButton = new JButton("Move", 32 new ImageIcon("hand.gif")); class MoveButtonListener implements ActionListener 35 { 36 public void actionPerformed(ActionEvent event) 37 {

38 int x = Integer.parseInt(xField.getText()); 39 int y = Integer.parseInt(yField.getText()); 40 box.setLocation(x, y); 41 repaint(); 42 } 43 }; ActionListener listener = new MoveButtonListener(); 46 moveButton.addActionListener(listener); // the labels for labeling the text fields 49 JLabel xLabel = new JLabel("x = "); 50 JLabel yLabel = new JLabel("y = "); // the panel for holding the user interface components 53 JPanel panel = new JPanel(); panel.add(xLabel); 56 panel.add(xField); 57 panel.add(yLabel); 58 panel.add(yField);

59 panel.add(moveButton); // the frame for holding the component panel 62 JFrame frame = new JFrame(); 63 frame.setContentPane(panel); 64 frame.pack(); 65 frame.show(); 66 } public void paint(Graphics g) 69 { 70 Graphics2D g2 = (Graphics2D)g; 71 g2.draw(box); 72 } 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 }

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

An Applet with Multiple Buttons

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; }

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));

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; /** 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()

18 { 19 // the rectangle that the paint method draws 20 box = new Rectangle(BOX_X, BOX_Y, 21 BOX_WIDTH, BOX_HEIGHT); // the panel for holding the user interface components 24 JPanel panel = new JPanel(); 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)); // the frame for holding the component panel 32 JFrame frame = new JFrame(); 33 frame.setContentPane(panel); 34 frame.pack(); 35 frame.show(); 36 } 37

38 public void paint(Graphics g) 39 { 40 Graphics2D g2 = (Graphics2D)g; 41 g2.draw(box); 42 } /** 45 Makes a button that moves the box. label the label to show on the button dx the amount by which to move the box in x-direction 48 when the button is clicked dy the amount by which to move the box in y-direction 50 when the button is clicked the button 52 */ 53 public JButton makeButton(String label, final int dx, 54 final int dy) 55 { 56 JButton button = new JButton(label); 57

58 class ButtonListener implements ActionListener 59 { 60 public void actionPerformed(ActionEvent event) 61 { 62 box.translate(dx, dy); 63 repaint(); 64 } 65 }; ButtonListener listener = new ButtonListener(); 68 button.addActionListener(listener); 69 return button; 70 } 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 }

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);

A Frame with Two Labels

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.EXIT_ON_CLOSE); JLabel iconLabel = new JLabel(new ImageIcon("world.gif")); 17 JLabel textLabel = new JLabel("Hello, World!");

18 19 JPanel panel = new JPanel(); 20 panel.add(iconLabel); 21 panel.add(textLabel); 22 frame.setContentPane(panel); frame.pack(); 25 frame.show(); 26 } 27 }

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);

A Text Area with Scroll Bars

The Control Panel for Adding Interest

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; /** 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 {

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); // 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(); // the label and text field for entering the interest rate 35 JLabel rateLabel = new JLabel("Interest Rate: "); final JTextField rateField = new JTextField(10);

38 rateField.setText("" + DEFAULT_RATE); // the button to trigger the calculation 41 JButton calculateButton = new JButton("Add Interest"); class CalculateListener implements ActionListener 44 { 45 public void actionPerformed(ActionEvent 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 } ActionListener listener = new CalculateListener(); 57 calculateButton.addActionListener(listener);

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); // the frame to hold the control panel 66 JFrame controlFrame = new JFrame(); 67 controlFrame.setContentPane(controlPanel); 68 controlFrame.pack(); 69 controlFrame.show(); 70 } private static final double DEFAULT_RATE = 10; 73 private static final double INITIAL_BALANCE = 1000; 74 }