如何将内部类写成lambda表达式？

Question

 public void actionPerformed(ActionEvent event) {
                
                red.addActionListener(e -> {
                    getContentPane().setBackground(Color.RED);
                });
                green.addActionListener(e -> {
                    getContentPane().setBackground(Color.GREEN);
                });
                blue.addActionListener(e -> {
                    getContentPane().setBackground(Color.BLUE);
                });
                clear.addActionListener(e -> {
                    getContentPane().setBackground(null);
                });
            }

在本期节目中：

package com.IST242Apps;

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

public class ColorFrame extends JFrame{
JButton red, green, blue, clear;

public ColorFrame() { //frame parameters
    super("ColorFrame");
    setSize(500,500);
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    FlowLayout flo = new FlowLayout();
    setLayout(flo);
    
    //create buttons
    red = new JButton("RED");
    add(red);
    green = new JButton("GREEN");
    add(green);
    blue = new JButton("BLUE");
    add(blue);
    clear = new JButton("CLEAR");
    add(clear);

    //Something starts here -- the inner class?
    ActionListener act = new ActionListener() { // 
         public void actionPerformed(ActionEvent event) { //when button is clicked
                
                red.addActionListener(e -> {
                    getContentPane().setBackground(Color.RED); //make red
                });
                green.addActionListener(e -> {
                    getContentPane().setBackground(Color.GREEN); //make green
                });
                blue.addActionListener(e -> {
                    getContentPane().setBackground(Color.BLUE); //make blue
                });
                clear.addActionListener(e -> {
                    getContentPane().setBackground(null); //make clear
                });
            }
        };
    //comment: something ends here
        
    //execute lambda expressions
    red.addActionListener(act); 
    green.addActionListener(act);
    blue.addActionListener(act);
    clear.addActionListener(act);
    
    setVisible(true); //make visible
    }
    public static void main(String args[]) {
        new ColorFrame();
    }
}

作为lambda表达式。然而，我对如何实际用lambda表达式替换内部类感到困惑，因为我以前从未这样做过。

如果您也能解释lambda表达式是如何工作的，就像我在W3Schools上看到的那样。

//numbers.foreach(n)->{system.out.println(n)；}）；

    ArrayList<Integer> numbers = new ArrayList<Integer>();
    numbers.add(5);
    numbers.add(9);
    numbers.add(8);
    numbers.add(1);
    numbers.forEach( (n) -> { System.out.println(n); } );

（1）你引用了一些东西。（数字）
(2)表示某种命令。(forEach)
(3)设置参数(n)
(4)表示一些要执行的代码。（{system.out.println(n)；}）
//n只是引用一个不存在的变量吗？

Answer 1

ActionListener act = event -> { //when button is clicked

    red.addActionListener(e -> {
        getContentPane().setBackground(Color.RED); //make red
    });
    green.addActionListener(e -> {
        getContentPane().setBackground(Color.GREEN); //make green
    });
    blue.addActionListener(e -> {
        getContentPane().setBackground(Color.BLUE); //make blue
    });
    clear.addActionListener(e -> {
        getContentPane().setBackground(null); //make clear
    });
};