import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class Calculator
{
JFrame window;
// stuff for top panel
JPanel topPanel;
JTextField expr,result;
JButton equals;

// stuff for bottom panel

JPanel bottomPanel,digitsPanel,opsPanel;
JButton[] digits,ops;
JButton clear, clearEntry;
Container content;
Listener listener;
String[] oplabels = { "+", "-", "/", "*" };

public Calculator()
{
    listener = new Listener(); // our Listener class implements ActionListener
    window= new JFrame("GUI Calc");
    content=window.getContentPane();
    content.setLayout( new GridLayout(2,1) );
    topPanel=new JPanel();
    topPanel.setLayout( new GridLayout(1,3) );

    // TOP PANEL WORK

    expr = new JTextField( );
    equals = new JButton("=");
    equals.addActionListener( listener );
    result = new JTextField( );

    topPanel.add( expr );
    topPanel.add( equals );
    topPanel.add( result );

    // BOTTOM PANEL WORK

    bottomPanel = new JPanel();
    bottomPanel.setLayout( new GridLayout(1,2) );

    digitsPanel = new JPanel();
    digitsPanel.setLayout( new GridLayout(4,3) );

    opsPanel = new JPanel();
    opsPanel.setLayout( new GridLayout(4,1) );

    digits  = new JButton[12];
    ops = new JButton[4];

    for (int i=0 ; i<10 ; i++)
    {
        digits[i] = new JButton( i+"" );
        digits[i].addActionListener(listener);
        digitsPanel.add( digits[i] );
    }

    clear = new JButton( "C" );
    clearEntry = new JButton( "CE" );
    clear.addActionListener(listener);
    clearEntry.addActionListener(listener);
    digitsPanel.add( clear );
    digitsPanel.add( clearEntry);


    for (int i=0 ; i<4 ; i++)
    {
        ops[i] = new JButton( oplabels[i] ) ;
        ops[i].addActionListener(listener);
        opsPanel.add( ops[i] );
    }

    bottomPanel.add( digitsPanel );
    bottomPanel.add( opsPanel );


    content.add( topPanel);
    content.add( bottomPanel);

    window.setVisible(true);
}
class Listener implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        Component clicked = (Component) e.getSource();
        if ( clicked == equals )
        {
            result.setText( evaluate( expr.getText() ) );
            return;
        }
        for ( int i=0 ; i<10 ; i++)
        {
            if ( clicked == digits[i] )
            {
                expr.setText( expr.getText() + i );
                return;
            }
        }
        if ( clicked == clear )
        {
            expr.setText("0");
            return;
            // do something

        }
/*          if (clicked == clearEntry)
        {
        expr.setText(expr.getDigits[i]);
        }
        for (int i=0; i < 10; i++)
        {
            if (clicked == digits[i])
            {
                int lastValue = clicked;
                if(clicked == clearEntry)
                {
                expr.setText(expr.getText()-clicked);
                return;
                }
            }
        } */

        for ( int i=0 ; i<4 ; i++ )
        {
            if(clicked == digits[i])
            {
            expr.setText(expr.getText() + oplabels[i]);
            return;
            }
            // tack on that operator to the expr string
        }

    }
    String evaluate( String exp )
    {
        return "NOT WRITTEN YET";
    }
}
public static void main(String [] args)
{
    new Calculator();
}
}

Component last_clicked;

last_clicked=null;

last_clicked=clicked;