A Simple Calculator

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 calculator panel
*/
class CalculatorFrame extends JFrame{
public CalculatorFrame(){
setTitle("Calculator");
//setSize(100, 100);
CalculatorPanel panel = new CalculatorPanel();
add(panel);
pack();
}
}

/*
*A panel with calulator buttons and a result display
*/
class CalculatorPanel extends JPanel{
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 in a 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.
*@param label the button label
*@param listener the button listener
*/
public void addButton(String label, ActionListener listener){
JButton button = new JButton(label);
button.addActionListener(listener);
panel.add(button);
}

/*
*Carries out the pending calculation.
*@param x the value to be accumulated with the prior result.
*/
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);
}

/*
*This action inserts the button action string to the end of the display text.
*/
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);
}
}

/*
*This action executes the command that the button action string denotes.
*/
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{
calculate(Double.parseDouble(display.getText()));
lastCommand = command;
start = true;
}
}
}

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