test.java

《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);
	}
}