事件4：点击一个按钮，改变按钮上面的字

import  java.awt. * ;
import  javax.swing. * ;
import  java.awt.event. * ;

public   class  actionTest  extends  JFrame  implements  ActionListener ... {
    public static void main(String s[])...{
        new actionTest();
    }
    public actionTest()...{
        JButton b=new JButton("ok");
        b.addActionListener(this);
        this.getContentPane().add(b);
        this.setSize(200,200);
        this.setVisible(true);
    }
    public void actionPerformed(ActionEvent e)...{
        JButton jb=(JButton)e.getSource();
        if(e.getActionCommand().equals("ok"))
            jb.setText("Clicked");
        else
            jb.setText("ok");
    }
}