Find Daniel explained , the program can not run , in the panel to achieve three button, click the corresponding color

transform

import java.awt.Color;
  import java.awt.EventQueue;
  import java.awt.event.ActionEvent;
  import java.awt.event.ActionListener;
  
  import javax.swing.JButton;
  import javax.swing.JFrame;
  import javax.swing.JPanel;
  
  
  public class Test {
  
  	/**
  	 * @param args
  	 */
  	public static void main(String[] args) {
  		EventQueue.invokeLater(new Runnable()
  		{
  			public void run()
  			{
  				TestFrame frame = new TestFrame();
  				frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  				frame.setVisible(true);
  			}
  		});
  
  	}
  	
  }
  class TestFrame extends JFrame 
  {
  	public TestFrame()
  	{
  		setTitle("First Swing Test");
  		setSize(FRAME_WIDTH,FRAME_HEIGHT);
  		
  		makeButton("yellow",Color.YELLOW);
  		makeButton("pink",Color.PINK);
  		makeButton("red",Color.RED);
  		
  	}
  	public void makeButton (String name,final Color backgroundColor)
  	{
  		JButton button = new JButton(name);
  		buttonPanel.add(button);
  
  		button.addActionListener(new ActionListener()
  		{
  			public void actionPerformed(ActionEvent event)
  			{
  				buttonPanel.setBackground(backgroundColor);
  			}
  		});
  	}		
  	
  	private JPanel buttonPanel;
  		
  	private static int FRAME_WIDTH = 350;
  	private static int FRAME_HEIGHT = 500;
  }
  
  
  
  

I did not run properly for anonymous class before then streamlined , set up a method makeButton, set up anonymous class to die , seeking Daniel explains , Jia
------ Solution --------------------------------------------

  
  class TestFrame extends JFrame {
  	private JPanel buttonPanel;
  
  	private static int FRAME_WIDTH = 350;
  	private static int FRAME_HEIGHT = 500;
  	/**
  	 * 
  	 */
  	private static final long serialVersionUID = 1L;
  
  	public TestFrame() {
  		setTitle("First Swing Test");
  		setSize(FRAME_WIDTH, FRAME_HEIGHT);
  
  		/**
  		 * 你写了一个 buttonPanel 变量，但是却没有给它赋值，所以出现了空指针的问题。 下面我给它进行了赋值，问题就解决了。
  		 */
  		buttonPanel = new JPanel();
  		add(buttonPanel);
  		makeButton("yellow", Color.YELLOW);
  		makeButton("pink", Color.PINK);
  		makeButton("red", Color.RED);
  
  	}
  
  	public void makeButton(String name, final Color backgroundColor) {
  		JButton button = new JButton(name);
  		buttonPanel.add(button);
  
  		button.addActionListener(new ActionListener() {
  			public void actionPerformed(ActionEvent event) {
  				buttonPanel.setBackground(backgroundColor);
  			}
  		});
  	}
  
  }
  

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landlord is beginner java bar , this problem will be very common in the future , I hope to learn and progress landlord