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;
}