监听JTextfield内容的改变

请看下面的代码：

package test;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;

public class test extends JFrame implements DocumentListener{
	public static void main(String[] args){
		new test().init();
	}
	
	JPanel contentPanel;
	JTextField textField;
	public void init(){
		contentPanel = new JPanel();
		setContentPane(contentPanel);
		contentPanel.setBounds(400,200,400,400);
		
		textField = new JTextField();
		textField.setBounds(0, 0, 100, 30);
		textField.setColumns(10);
		
		//关键是下面这两行代码
		Document document = textField.getDocument();
		document.addDocumentListener(this);
		
		contentPanel.add(textField);
		
		this.setVisible(true);
		this.setSize(500, 400);
		this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	}

	@Override
	public void insertUpdate(DocumentEvent e) {
		System.out.println("insert text");
	}

	@Override
	public void removeUpdate(DocumentEvent e) {
		System.out.println("remove text");
	}

	@Override
	public void changedUpdate(DocumentEvent e) {
		System.out.println("change text");
	}
}