Calculator

/**
@version 1.32 2004-05-05

*/

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

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

/**
A frame with a calculator panel.
*/
class CalculatorFrame extends JFrame
{
public CalculatorFrame()
{
   setTitle("Calculator");
   CalculatorPanel panel = new CalculatorPanel();
   add(panel);
 pack();
}
}

/**
A panel with calculator 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 x 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
*/
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
{
   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;
      }
   }
}

/**
   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);
}

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