这是一个改变背景颜色的程序，但运行后颜色没改变阿？
请帮忙看看。谢谢

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

class FirstPanel extends JPanel
      implements ActionListener
{     public FirstPanel()
      {      JButton redButton = new JButton("Red");
             JButton blueButton = new JButton("Blue");
             JButton yellowButton = new JButton("Yellow");

             add(redButton);
             add(blueButton);
             add(yellowButton);

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

      public void actionPerformed(ActionEvent evt)
      {     Object source = evt.getSource();
            Color color = getBackground();
            if (source == redButton) color = Color.red;
            else if (source == blueButton) color = Color.blue;
            else if (source == yellowButton) color = Color.yellow;
            setBackground(color);
            repaint();
      }
      private JButton redButton;
      private JButton blueButton;
      private JButton yellowButton;
}
class FirstFrame extends JFrame
{     public FirstFrame()
      {      setTitle("FirstFrame");
             setSize(300, 200);
             addWindowListener(new WindowAdapter()
             {    public void windowClosing(WindowEvent e)
                  {      System.exit(0);
                  }
             });
         
          Container contentPane = getContentPane();
          contentPane.add(new FirstPanel());
      }
}

public class FirstTest
{      public static void main(String[] args)
       {      JFrame frame = new FirstFrame();
              frame.setVisible(true);
       }
}

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

class FirstPanel extends JPanel
      implements ActionListener
{     public FirstPanel()
      {      JButton redButton = new JButton("Red");//你在这个地方不应该再定义三个JButton，你把这三个button前的JButton去掉就可以了！你试试！
             JButton blueButton = new JButton("Blue");
             JButton yellowButton = new JButton("Yellow");

             add(redButton);
             add(blueButton);
             add(yellowButton);

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

      public void actionPerformed(ActionEvent evt)
      {     Object source = evt.getSource();
            Color color = getBackground();
            if (source == redButton) color = Color.red;
            else if (source == blueButton) color = Color.blue;
            else if (source == yellowButton) color = Color.yellow;
            setBackground(color);
            repaint();
      }
      private JButton redButton;
      private JButton blueButton;
      private JButton yellowButton;
}
class FirstFrame extends JFrame
{     public FirstFrame()
      {      setTitle("FirstFrame");
             setSize(300, 200);
             addWindowListener(new WindowAdapter()
             {    public void windowClosing(WindowEvent e)
                  {      System.exit(0);
                  }
             });
         
          Container contentPane = getContentPane();
          contentPane.add(new FirstPanel());
      }
}

public class FirstTest
{      public static void main(String[] args)
       {      JFrame frame = new FirstFrame();
              frame.setVisible(true);
       }
}

错误在于
      private JButton redButton;
      private JButton blueButton;
      private JButton yellowButton;
然后构造器中再
             JButton redButton = new JButton("Red");
             JButton blueButton = new JButton("Blue");
             JButton yellowButton = new JButton("Yellow");
明白 ? 一开始偶也被迷惑了. . . . .