小学德育实践活动总结:一个java程序-要求当点按钮时，Label上的字符会相应改变。出了些问题....程序如下：

import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
public class First extends JPanel
{
private JButton btn;
private JLabel j;
public First()
{
setLayout(new BoxLayout(this,BoxLayout.Y_AXIS));
btnListener l=new btnListener();
btn.addActionListener(l);
btn=new JButton("click");
j=new JLabel("ok");
j.setBackground(Color.YELLOW);
add(btn);
add(j);
}
private class btnListener implements ActionListener
{
public void actionPerformed(ActionEvent event)
{
if(event.getSource()==btn)
j.setText("Cancel");
else
j.setText("That's all!");
}
}
}
主程序:import javax.swing.JFrame;

public class Driver {

public static void main(String[] args)
{
JFrame frm=new JFrame();
frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE );

First f=new First();
frm.getContentPane().add(f);
frm.pack();
frm.setVisible(true);

}

}