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

class javaButton
{
public static void main(String[] args)
{
ButtonTestFrame frame = new ButtonTestFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.show();
}
}

class ButtonTestFrame extends JFrame
{
public ButtonTestFrame()
{
setTitle("Button");
setSize(200, 400);

 ButtonPanel panel = new ButtonPanel();
 Container contentPane = getContentPane();
 contentPane.add(panel);
 }
}

class ButtonPanel extends JPanel
{
public ButtonPanel()
{
JButton yellowButton = new JButton("Yellow");
add(yellowButton);
ColorAction yellowAction = new ColorAction(Color.yellow);
yellowButton.addActionListener(yellowAction);
}


}

class ColorAction implements ActionListener
{
public ColorAction(Color back)
{
backgroundColor = back;
}

public void actionPerformed(ActionEvent event)
{
bpane.setBackground(backgroundColor);
bpane.repaint();
}

private Color backgroundColor;
private ButtonPanel bpane;
}
这是我的代码，编译通过，执行时出现异常，

代码修改如下：
自己分析原因

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

class javaButton
{
        public static void main(String[] args)
        {
                ButtonTestFrame frame = new ButtonTestFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.show();
        }
}

class ButtonTestFrame extends JFrame
{
        public ButtonTestFrame()
        {
                setTitle("Button");
                setSize(200, 400);

                 ButtonPanel panel = new ButtonPanel();
                 Container contentPane = getContentPane();
                 contentPane.add(panel);
         }
}

class ButtonPanel extends JPanel
{
        public ButtonPanel()
        {
                JButton yellowButton = new JButton("Yellow");
                add(yellowButton);
                ColorAction yellowAction = new ColorAction(Color.yellow,this);
                yellowButton.addActionListener(yellowAction);
        }


}

class ColorAction implements ActionListener
{
        public ColorAction(Color back,JPanel backpane)
        {
                backgroundColor = back;
                bpane = (ButtonPanel)backpane;
        }

        public void actionPerformed(ActionEvent event)
        {

                bpane.setBackground(backgroundColor);
                bpane.repaint();
        }

        private Color backgroundColor;
        private ButtonPanel bpane;
}