哪里错了，谢谢。20分

[freezhATsis]

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

public class ButtonTest{

public static void main(String[] args){
ButtonFrame frame = new ButtonFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}

}

class ButtonFrame extends JFrame{

private JPanel buttonPanel;
private JButton yellowButton;
private JButton redButton;
private JButton blueButton;

public ButtonFrame(){
setTitle("ButtonTest");
setSize(300,200);
yellowButton = new JButton("Yellow");
redButton = new JButton("Red");
blueButton = new JButton("Blue");
buttonPanel = new JPanel();

buttonPanel.add(yellowButton);
buttonPanel.add(blueButton);
buttonPanel.add(redButton);

add(buttonPanel);

yellowButton.addActionListener(this);
redButton.addActionListener(this);
blueButton.addActionListener(this);
}
public void actionPerformed(ActionEvent e){
Object eventSource = e.getSource();
if(eventSource == yellowButton){
buttonPanel.setBackground(Color.yellow);
}else if(eventSource == redButton){
buttonPanel.setBackground(Color.red);
}else if(eventSource == blueButton){
buttonPanel.setBackground(Color.blue);
}
}
}

[yuyeyi]

没有加入监听器接口加implements

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

public class ButtonTest{ 
  
  public static void main(String[] args){ 
    ButtonFrame frame = new ButtonFrame(); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frame.setVisible(true); 
  } 
  
} 

class ButtonFrame extends JFrame{ 
  
  private JPanel buttonPanel; 
  private JButton yellowButton; 
  private JButton redButton; 
  private JButton blueButton; 
  
  public ButtonFrame(){ 
    setTitle("ButtonTest"); 
    setSize(300,200); 
    yellowButton = new JButton("Yellow"); 
    redButton = new JButton("Red"); 
    blueButton = new JButton("Blue"); 
    buttonPanel = new JPanel(); 
    
    buttonPanel.add(yellowButton); 
    buttonPanel.add(blueButton); 
    buttonPanel.add(redButton); 
    
    add(buttonPanel); 
  
    yellowButton.addActionListener(this); 
    redButton.addActionListener(this); 
    blueButton.addActionListener(this); 
  } 
    public void actionPerformed(ActionEvent e){ 
      Object eventSource = e.getSource(); 
      if(eventSource == yellowButton){ 
        buttonPanel.setBackground(Color.yellow); 
        }else if(eventSource == redButton){ 
          buttonPanel.setBackground(Color.red); 
      }else if(eventSource == blueButton){ 
        buttonPanel.setBackground(Color.blue); 
      } 
    } 
  } 
    

[yuyeyi]

上面的粘贴到你错误的
不好意思

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

public class ButtonTest{ 
  
  public static void main(String[] args){ 
    ButtonFrame frame = new ButtonFrame(); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frame.setVisible(true); 
  } 
  
} 

class ButtonFrame extends JFrame implements ActionListener{ 
  
  private JPanel buttonPanel; 
  private JButton yellowButton; 
  private JButton redButton; 
  private JButton blueButton; 
  
  public ButtonFrame(){ 
    setTitle("ButtonTest"); 
    setSize(300,200); 
    yellowButton = new JButton("Yellow"); 
    redButton = new JButton("Red"); 
    blueButton = new JButton("Blue"); 
    buttonPanel = new JPanel(); 
    
    buttonPanel.add(yellowButton); 
    buttonPanel.add(blueButton); 
    buttonPanel.add(redButton); 
    
    add(buttonPanel); 
  
    yellowButton.addActionListener(this); 
    redButton.addActionListener(this); 
    blueButton.addActionListener(this); 
  } 
    public void actionPerformed(ActionEvent e){ 
      Object eventSource = e.getSource(); 
      if(eventSource == yellowButton){ 
        buttonPanel.setBackground(Color.yellow); 
        }else if(eventSource == redButton){ 
          buttonPanel.setBackground(Color.red); 
      }else if(eventSource == blueButton){ 
        buttonPanel.setBackground(Color.blue); 
      } 
    } 
  } 
    

[lzbang]

yellowButton.addActionListener(this);
redButton.addActionListener(this);
blueButton.addActionListener(this);

没有声明监听接口

[yuyeyi]

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

public class ButtonTest{

public static void main(String[] args){
ButtonFrame frame = new ButtonFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}

}

class ButtonFrame extends JFrame implements ActionListener{

private JPanel buttonPanel;
private JButton yellowButton;
private JButton redButton;
private JButton blueButton;

public ButtonFrame(){
setTitle("ButtonTest");
setSize(300,200);
yellowButton = new JButton("Yellow");
redButton = new JButton("Red");
blueButton = new JButton("Blue");
buttonPanel = new JPanel();

buttonPanel.add(yellowButton);
buttonPanel.add(blueButton);
buttonPanel.add(redButton);

add(buttonPanel);

yellowButton.addActionListener(this);
redButton.addActionListener(this);
blueButton.addActionListener(this);
}
public void actionPerformed(ActionEvent e){
Object eventSource = e.getSource();
if(eventSource == yellowButton){
buttonPanel.setBackground(Color.yellow);
}else if(eventSource == redButton){
buttonPanel.setBackground(Color.red);
}else if(eventSource == blueButton){
buttonPanel.setBackground(Color.blue);
}
}
}

[lzbang]

import java.awt.*; 
import java.awt.event.*; 

import javax.swing.*; 

public class ButtonTest{ 
  
  public static void main(String[] args){ 
    ButtonFrame frame = new ButtonFrame(); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    frame.setVisible(true); 
  } 
  
} 

class ButtonFrame extends JFrame{ 
  
  private JPanel buttonPanel; 
  private JButton yellowButton; 
  private JButton redButton; 
  private JButton blueButton; 
  
  public ButtonFrame(){ 
    setTitle("ButtonTest"); 
    setSize(300,200); 
    yellowButton = new JButton("Yellow"); 
    redButton = new JButton("Red"); 
    blueButton = new JButton("Blue"); 
    buttonPanel = new JPanel(); 
    
    buttonPanel.add(yellowButton); 
    buttonPanel.add(blueButton); 
    buttonPanel.add(redButton); 
    
    add(buttonPanel); 
  
    yellowButton.addActionListener(new ActionListener(){
		public void actionPerformed(ActionEvent ae){
			buttonPanel.setBackground(Color.yellow); 
		}
	});
    
    redButton.addActionListener(new ActionListener(){
		public void actionPerformed(ActionEvent ae){
			 buttonPanel.setBackground(Color.red); 
		}
	});
    blueButton.addActionListener(new ActionListener(){
		public void actionPerformed(ActionEvent ae){
			 buttonPanel.setBackground(Color.blue); 
		}
	});
  } 
    
  } 
    

[freezhATsis]

恍然大悟，谢谢了！

[pspyan]

class ButtonFrame extends JFrame implements ActionListener
实现ActionListener