15.10. Scroll Bars
14.5. Key Events
Key events enable the use of the keys to control and perform actions or get input from the keyboard. A key event is fired whenever a key is pressed, released, or typed on a component. The KeyEvent object describes the nature of the event (namely, that a key has been pressed, released, or typed) and the value of the key, as shown in Figure 14.13. Java provides the KeyListener to handle key events, as shown in Figure 14.14.
Figure 14.13. The KeyEvent class encapsulates information about key events.
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Figure 14.14. The KeyListener interface handles key pressed, released, and typed events.
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The keyPressed handler is invoked when a key is pressed, the keyReleased handler is invoked when a key is released, and the keyTyped handler is invoked when a Unicode character is entered. If a key does not have a Unicode (e.g., function keys, modifier keys, action keys, and control keys), the keyTyped handler will be not be invoked.
Every key event has an associated key character or key code that is returned by the getKeyChar() or getKeyCode() method in KeyEvent . The key codes are constants defined in Table 14.4. For a key of the Unicode character, the key code is the same as the Unicode value. For the key pressed and key released events, getKeyCode() returns the value as defined in the table. For the key typed event, getKeyCode() returns VK_UNDEFINED .
Table 14.4. Key Constants
| Constant | Description |
|---|---|
| VK_HOME | The Home key |
| VK_END | The End key |
| VK_PGUP | The Page Up key |
| VK_PGDN | The Page Down key |
| VK_UP | The up-arrow key |
| VK_DOWN | The down-arrow key |
| VK_LEFT | The left-arrow key |
| VK_RIGHT | The right-arrow key |
| VK_ESCAPE | The Esc key |
| VK_TAB | The Tab key |
| VK_CONTROL | The Control key |
| VK_SHIFT | The Shift key |
| VK_BACK_SPACE | The Backspace key |
| VK_CAPS_LOCK | The Caps Lock key |
| VK_NUM_LOCK | The Num Lock key |
| VK_ENTER | The Enter key |
| VK_UNDEFINED | The keyCode unknown |
| VK_F1 to VK_F12 | The function keys from F1 to F12 |
| VK_0 to VK_9 | The number keys from 0 to 9 |
| VK_A to VK_Z | The letter keys from A to Z |
The program in Listing 14.8 displays a user-input character. The user can move the character up, down, left, and right, using the arrow keys VK_UP , VK_DOWN , VK_LEFT , and VK_RIGHT . Figure 14.15 contains a sample run of the program.
Figure 14.15. The program responds to key events by displaying a character and moving it up, down, left, or right.
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Listing 14.8. KeyEventDemo.java
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1 import java.awt.*; 2 import java.awt.event.*; 3 import javax.swing.*; 4 5 public class KeyEventDemo extends JFrame { 6 private KeyboardPanel keyboardPanel = new KeyboardPanel(); 7 8 /** Initialize UI */ 9 public KeyEventDemo() { 10 // Add the keyboard panel to accept and display user input 11 add(keyboardPanel); 12 13 // Set focus 14 keyboardPanel.setFocusable( true ); 15 } 16 17 /** Main method */ 18 public static void main(String[] args) { 19 KeyEventDemo frame = new KeyEventDemo(); 20 frame.setTitle( "KeyEventDemo" ); 21 frame.setLocationRelativeTo( null ); // Center the frame 22 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 23 frame.setSize( 300 , 300 ); 24 frame.setVisible( true ); 25 } 26 27 // Inner class: KeyboardPanel for receiving key input 28 static class KeyboardPanel extends JPanel { 29 private int x = 100 ; 30 private int y = 100 ; 31 private char keyChar = 'A' ; // Default key 32 33 public KeyboardPanel() { 34 addKeyListener( new KeyAdapter() { 35 public void keyPressed(KeyEvent e) { 36 switch (e.getKeyCode()) { 37 case KeyEvent.VK_DOWN: y += 10 ; break ; 38 case KeyEvent.VK_UP: y -= 10 ; break ; 39 case KeyEvent.VK_LEFT: x -= 10 ; break ; 40 case KeyEvent.VK_RIGHT: x += 10 ; break ; 41 default: keyChar = e.getKeyChar(); 42 } 43 44 repaint(); 45 } 46 } ); 47 } 48 49 /** Draw the character */ 50 protected void paintComponent(Graphics g) { 51 super .paintComponent(g); 52 53 g.setFont( new Font( "TimesRoman" , Font.PLAIN, 24 )); 54 g.drawString(String.valueOf(keyChar), x, y); 55 } 56 } 57 } |
The KeyboardPanel class extends JPanel to display a character (line 28). This class is declared as an inner class inside the main class because it is only used in this class. Furthermore, the inner class is declared 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 non-arrow key is pressed, the key is displayed (line 41). When an arrow key is pressed, the character moves in the direction indicated by the arrow key (lines 37 “40).
Only a focused component can receive KeyEvent . To make a component focusable , set its isFocusable property to true (line 14).
Every time the component is repainted, a new font is created for the Graphics object in line 53. This is not efficient. It is better to create the font one time as a data field.
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