为什么会错呢
import java.awt.event.*;
import javax.swing.JFrame;

public class TestKey {
public static void main(String[]args){
new TestKey().launchFrame();
}

void launchFrame(){
addKeyListener(new Monitor());
setBounds(300,300,300,500);
setVisible(true);

setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}

class Monitor extends KeyAdapter{
public void keyPressed(KeyEvent e){
int keycode=e.getKeyCode();
if(keycode==KeyEvent.VK_0 ){
System.out.println("nihao");
}
}
}
}
[最优解释]
没有extends JFrame

public class TestKey extends JFrame