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 
{ 
  public CalculatorFrame() 
  { 
    setTitle("Calculator"); 
    CalculatorPanel panel=new CalculatorPanel(); 
    add(panel); 
    pack(); 
  } 
} 
 
 
class CalculatorPanel extends JPanel 
{ 
  private JButton display; 
    private JPanel panel; 
    private double result; 
    private String lastCommand; 
    private boolean start; 
  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(); 
     
    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); 
     
    } 
  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); 
  } 
}