JAVA编程题:创建有一个文本框和三个按钮的小程序。当按下每个按钮时，使不同的文字显示在文本框中

import javax.swing.JPanel;
import javax.swing.JFrame;
import javax.swing.JButton;
import javax.swing.JTextField;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

public class Test extends JFrame implements ActionListener{

private static final long serialVersionUID = 1L;
private JPanel jPanel;
private JButton jButton1,jButton2,jButton3;
private JTextField tf1;

public Test(String title) {
super(title);
init();
}

private void init() {
jPanel=new JPanel();
jPanel.setLayout(new FlowLayout());
tf1=new JTextField(20);
jButton1=new JButton("按钮1");
jButton2=new JButton("按钮2");
jButton3=new JButton("按钮3");
jButton1.addActionListener(this);
jButton2.addActionListe