EVENT-DRIVEN PROGRAMMING - Introduction to Java Programming - page 623

29 private int x = 100 ;

30 private int y = 100 ;

31 private char keyChar = 'A' ; // Default key

32
33 public KeyboardPanel() {
34 addKeyListener(
35 @Override
36
37

new KeyAdapter() { register listener

public void keyPressed(KeyEvent e) { override handler

switch (e.getKeyCode()) {

38 case KeyEvent.VK_DOWN: y += 10 ; break ;

39 case KeyEvent.VK_UP: y -= 10 ; break ;

40 case KeyEvent.VK_LEFT: x -= 10 ; break ;

41 case KeyEvent.VK_RIGHT: x += 10 ; break ;

42 default : keyChar = e.getKeyChar();

get the key pressed

43 }
44
45 repaint();
46 }
47 );
48 }
49
50 @Override /** Draw the character */
51

repaint

}

protected void paintComponent(Graphics g) {

52 super .paintComponent(g);

53
54 g.setFont( new Font( "TimesRoman" , Font.PLAIN, 24 ));
55 g.drawString(String.valueOf(keyChar), x, y);
56 }
57 }
58 }

redraw character

The KeyboardPanel class extends JPanel to display a character (line 28). This class is
defined as an inner class inside the main class, because it is used only in this class. Further-
more, the inner class is defined as static, because it does not reference any instance members
of the main class.
Because the program gets input from the keyboard, it listens for KeyEvent and extends
KeyAdapter to handle key input (line 34).
When a key is pressed, the keyPressed handler is invoked. The program uses
e.getKeyCode() to obtain the key code and e.getKeyChar() to get the character for the
key. When a nonarrow key is pressed, the character is displayed (line 42). When an arrow key
is pressed, the character moves in the direction indicated by the arrow key (lines 38-41).
Only a focused component can receive KeyEvent . To make a component focusable, set its
focusable property to true (line 14).
Every time the component is repainted, a new font is created for the Graphics object in
line 54. This is not efficient—it would be better to create the font once as a data field.
We can now add more control for our ControlCircle example in Listing 16.3 to
increase/decrease the circle radius by clicking the left/right mouse button or by pressing the
UP and DOWN arrow keys. The new program is given in Listing 16.10.

focusable

efficient?

L ISTING 16.10 ControlCircleWithMouseAndKey.java
1 import javax.swing.*;
2 import java.awt.*;
3 import java.awt.event.*;
4
5 public class ControlCircleWithMouseAndKey extends JFrame {
6 private JButton jbtEnlarge = new JButton( "Enlarge" );

7 private JButton jbtShrink = new JButton( "Shrink" );