简单的计算器程序，不知道哪错了，求大神指点！

Question

package calculator;

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

public class CalculatorPanel {

/**
 * @param args
 */
public static void main(String[] args) {
    // TODO Auto-generated method stub
    EventQueue.invokeLater(new Runnable()
       {
           public void run()
           {
               JFrame frame=new CalculatorPanel();
               frame.setTitle("Calculator");
               frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               frame.setVisible(true);
           }
       });
}

}
class CalculatorPanel extends JPanel
{
private JButton display;
private JPanel panel;
private double result;
private String lastCommand;
private boolean start;

public CalculatorPanel()
{
    setLayout(new BorderLayout());
    result=0;
    lastCommand="=";
    start=true;
    display=new JButton("0");
    display.setEnable(false);
    add(display,BorderLayout.NORTH);

    ActionListener insert=new InsertAction();
    ActionListener command=new CommandAction();

    panel=new JPanel();
    panel.setLayout(new GridLayout(4,4));

    addButton("7",insert);
    addButton("8",insert);
    addButton("9",insert);
    addButton("/",command);

    addButton("4",insert);
    addButton("5",insert);
    addButton("6",insert);
    addButton("*",command);

    addButton("1",insert);
    addButton("2",insert);
    addButton("3",insert);
    addButton("-",command);

    addButton("0",insert);
    addButton(".",insert);
    addButton("=",insert);
    addButton("+",command);

    add(panel,BorderLayout.CENTER);
}
private void addButton(String label,ActionListner listener)
{
    JButton button=new JButton(label);
    button.addActionListener(listener);
    panel.add(button);
}
private class InsertAction implements ActionListener
{
    public void actionPerformed(ActionEvent event)
    {
        String input=event.getActionCommand();
        if(start)
        {
            display.setText("");
            start=false;
        }
        display.setText(display.getText()+input);
    }
}
private class CommandAction implements ActionListener
{
    public void actionPerformed(ActionEvent event)
    {
        String command=event.getActionCommand();
        if(start)
        {
            if(command.equals("-"))
            {
                display.setText(command);
                start=false;
            }
            else lastCommand=command;
        }
        else 
        {
            calculator(Double.parseDouble(display.getText()));
            lastCommand=command;
            start=true;
        }
    }   
}
public void calculate(double x)
{
    if(lastCommand.equals("+")) result+=x;
    else if(lastCommand.equals("-")) result-=x;
    else if(lastCommand.equals("*")) result*=x;
    else if(lastCommand.equals("/")) result/=x;
    else if(lastCommand.equals("=")) result=x;
    display.setText(""+result);
}

}
错误提示说CalculatorPanel已经被定义了，为什么啊？怎么改啊？

Answer 1

public class CalculatorPanel {//这里定义了一个CalculatorPanel
/**
 * @param args
 */
public static void main(String[] args) {
    // TODO Auto-generated method stub
 -------------------------------------
 class CalculatorPanel extends JPanel   //这里又定义了一个CalculatorPanel
 这两个有一个改个名字