《Java2图形设计卷II：Swing》配套光盘源码

				import javax.swing.*;
				import javax.swing.text.*;
				import java.awt.*;
				import java.awt.event.*;
				
				public class Test extends JApplet {
					JTextField tf = new JTextField(3);
				
					public Test() {
						Container contentPane = getContentPane();
						JLabel label = new JLabel("Enter an Integer:");
				
						tf.setDocument(new IntegerDocument());
						
						contentPane.setLayout(new FlowLayout());	
						contentPane.add(label);
						contentPane.add(tf);
					}
				}
				class IntegerDocument extends PlainDocument {
					public void insertString(int offset, String s, 
											AttributeSet attributeSet) 
											throws BadLocationException {
						try {
							Integer.parseInt(s);
						}
						catch(Exception ex) { // only allow integer values
							Toolkit.getDefaultToolkit().beep();
							return;  
						}
						super.insertString(offset, s, attributeSet);
					}
				}