اليكم برنامج الالة الحاسبة

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Cal
{
	public static void main(String[] arg)
	{
		CalculatorFrame frame = new CalculatorFrame();
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.show();
	}
}
class CalculatorFrame extends JFrame
{
	public CalculatorFrame()
	{
		setTitle("Calculator");
		setSize(WIDTH,HEIGTH);
		setLocation(200,200);
		Container contentPane = getContentPane();
		CalculatorPanel panel = new CalculatorPanel();
		contentPane.add(panel);
	}
	public static final int WIDTH = 200;
	public static final int HEIGTH = 200;
}
class CalculatorPanel extends JPanel
{
	public CalculatorPanel()
	{
		setLayout(new BorderLayout());
		result = 0;
		lastCommand = "=";
		start = true;
		display = new JTextField("0");
		display.setEditable(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,ActionListener 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 evt)
    	{
    		String command = evt.getActionCommand();
    		if(start)
    		{
    			if(command.equals("-"))
     			{
    				display.setText(command);
    				start = false;
    			}
    			else
    			     lastCommand = command;
    		}
    		else
    		{
    			calculate(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);
     }

     private JTextField display;
     private JPanel panel;
     private double result;
     private String lastCommand;
     private boolean start;
}