小弟是菜鸟，请问下

Question

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;public class ButtenTest{

public static void main(String args[]){
JFrame frame=new JFrame("First Button");
Container contentPane=frame.getContentPane();
JButton b=new JButton("My First Button");
ActionListener listener=new ActionListener();//这里出错
b.addActionListener(listener);
b.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent actionEvent){
System.out.println("hey,stop that");
}
});
contentPane.add(b,BorderLayout.NORTH);
frame.setSize(300,200);
frame.show();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}编译时出错......
请问ActionListener listener=new ActionListener();  这样的语句是否合法？

Answer 1

非法
ActionListener是接口，怎么能实例化呢
b.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent actionEvent){
System.out.println("hey,stop that");
}
});
这里其实是一个无名内部类实现了ActionListener

Answer 2

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;public class ButtenTest{

public static void main(String args[]){
JFrame frame=new JFrame("First Button");
Container contentPane=frame.getContentPane();
JButton b=new JButton("My First Button");

                  //有了后面的内部类就好了，这里删去，2楼正解
                  //ActionListener listener=new ActionListener();//这里出错
//b.addActionListener(listener); b.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent actionEvent){
System.out.println("hey,stop that");
}
});
contentPane.add(b,BorderLayout.NORTH);
frame.setSize(300,200);
frame.show();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
}
}