JAVA计算器——摘自Java核心技术

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

/*
 * 
 */

public class Calculator 
{
    public static void main(String[] args)
    {
    	EventQueue.invokeLater(new Runnable()
    	{
    		public void run()
    		{
    			CalculatorFrame frame = new CalculatorFrame();
    			frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    			frame.setVisible(true);
    		}
    	});
    }
}

/*
 * A frame with a calculator panel
 */
class CalculatorFrame extends JFrame
{
	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;

	public CalculatorFrame()
	{
		setTitle("Calculator");
		CalculatorPanel panel = new CalculatorPanel();
		
		add(panel);
		pack();
	}
}

/*
 * A panel with calculator buttons and a result display.
 */
class CalculatorPanel extends JPanel
{
	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	public CalculatorPanel()
	{
		setLayout(new BorderLayout());
		
		result = 0;
		lastCommand = "=";
		start = true;
		
		//add the display
		
		display = new JButton("0");
		display.setEnabled(false);
		add(display, BorderLayout.NORTH);
		
		ActionListener insert = new InsertAction();
		ActionListener command = new CommandAction();
		
		//add the buttons with 4 * 4 grid
		
		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("=", command);
		addButton("+", command);
		
		add(panel, BorderLayout.CENTER);
		
	}
	
	/*
	 * adds a button to the center panel
	 */
	private void addButton(String label, ActionListener listener)
	{
		JButton button = new JButton(label);
		button.addActionListener(listener);
		panel.add(button);
	}
	
	/*
	 * This action inserts the button action string to the end of the display text.
	 */
	
	private class InsertAction implements ActionListener
	{

		@Override
		public void actionPerformed(ActionEvent event) {
			// TODO Auto-generated method stub
			String input = event.getActionCommand();
			if(start)
			{
				display.setText("");
				start = false;
			}
			display.setText(display.getText() + input);
		}
		
	}
	
	/*
	 * This action executes the command that the button action string denotes
	 */
	private class CommandAction implements ActionListener
	{

		@Override
		public void actionPerformed(ActionEvent event) {
			// TODO Auto-generated method stub
			String command = event.getActionCommand();
			
			if(start)
			{
				if(command.equals("-"))
				{
					display.setText(command);
					start = false;
				}
				else
					lastCommand = command;
			}
			else
			{
				calculate(Double.parseDouble(display.getText()));
				lastCommand = command;
				start = true;
			}
		}
		
	}
	
	/*
	 * Carries out the pending calculation
	 */
	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 JButton display;
	private JPanel panel;
	private double result;
	private String lastCommand;
	private boolean start;	
}