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

public class Calc2
{  
    public static void main(String[] args) 
    {
	SwingApp app = new SwingApp(250, 250);	    // set app window size
	app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    }
}

class SwingApp extends JFrame implements ActionListener
{
    private final DrawPanel panel;		    // class variable
    private final JButton b0, b1, b2, b3, b4, b5, b6, b7,  b8, b9;
    private final JButton add, sub, mul, div, equ, dot;  // operation buttons
    private final JTextField tf;                         // text field
    private String op;                             // storage for operation
    private double n1, n2;                         // storage for operands
 
    public SwingApp(int width, int height)	    // class constructor
    {
	super();				    // call to super class constructor
	Container pane = super.getContentPane();    // create layout, set colors
	panel = new DrawPanel();		    // add GUI components
	panel.setLayout(new BorderLayout(10, 10));	
	panel.setBackground(Color.yellow);
	Font font = new Font("Arial", Font.BOLD, 24); // text field font
	panel.setBackground(new Color(3, 163, 227));
	
	tf = new JTextField("0");                     // text field init
	tf.setFont(font);
	tf.setHorizontalAlignment(JTextField.RIGHT);
	panel.add(tf, BorderLayout.NORTH);

	JPanel jp = new JPanel();                     // JPanel with buttons
	jp.setLayout(new GridLayout(4, 4, 10, 10));
	b7 = new JButton("7");                        // create buttons
	b8 = new JButton("8");
	b9 = new JButton("9");
	add = new JButton("+");
	b4 = new JButton("4");
	b5 = new JButton("5");
	b6 = new JButton("6");
	sub = new JButton("-");
	b1 = new JButton("1");
	b2 = new JButton("2");
	b3 = new JButton("3");
	mul = new JButton("*");
	b0 = new JButton("0");
	dot = new JButton(".");
	equ = new JButton("=");
	div = new JButton("/");

	jp.add(b7);                                   // add buttons to the panel
	jp.add(b8);
	jp.add(b9);
	jp.add(add);
	jp.add(b4);
	jp.add(b5);
	jp.add(b6);
	jp.add(sub);
	jp.add(b1);
	jp.add(b2);
	jp.add(b3);
	jp.add(mul);
	jp.add(b0);
	jp.add(dot);
	jp.add(equ);
	jp.add(div);

	jp.setBackground(Color.ORANGE);
	panel.add(jp, BorderLayout.CENTER);

	b0.addActionListener(this);                  // register buttons for
	b1.addActionListener(this);                  // the event listener
	b2.addActionListener(this);
	b3.addActionListener(this);
	b4.addActionListener(this);
	b5.addActionListener(this);
	b6.addActionListener(this);
	b7.addActionListener(this);
	b8.addActionListener(this);
	b9.addActionListener(this);
	add.addActionListener(this);
	sub.addActionListener(this);
	mul.addActionListener(this);
	div.addActionListener(this);
	equ.addActionListener(this);
	dot.addActionListener(this);	
	
	pane.add(panel);	
	Toolkit toolkit = Toolkit.getDefaultToolkit();	// optionally position JFrame
	Dimension screenSize = toolkit.getScreenSize(); // in the middle of the screen 
	super.setLocation((screenSize.width - width)/2, (screenSize.height - height)/2);	
	super.setTitle("Swing app");		    // set desired window title
	super.setSize(width, height);		    // set desired window size
	super.setVisible(true);	
    }
    
    @Override
    public void actionPerformed(ActionEvent e)
    {
	if (e.getSource() == add || e.getSource() == sub ||  // process operation
	    e.getSource() == mul || e.getSource() == div)	 // button	
	{
	    n1 = Double.parseDouble(tf.getText());    // get 1st operand
	    tf.setText("0");                          // zero the text field
	    op = ((JButton) e.getSource()).getText(); // save operation
	}
	else if (e.getSource() == equ)              // the "=" button processing
	{
	    n2 = Double.parseDouble(tf.getText());    // get 2nd operand
	    String result = "";
	    if (op.equals("+"))                       // process "+" button
	    {
		result = (n1 + n2) + "";                // compute the result
		if (result.charAt(result.length()-2) == '.' && // get rid of tailing
		    result.charAt(result.length()-1) == '0')   // .0 for integer numbers
		    result = result.substring(0, result.length()-2);
		tf.setText(result);                     // show the result in text field
	    }
	    else if (op.equals("-"))                  // similarly process the "-"
	    {
		result = (n1 - n2) + "";
		if (result.charAt(result.length()-2) == '.' &&
		    result.charAt(result.length()-1) == '0')
		    result = result.substring(0, result.length()-2);
		tf.setText(result);
	    }
	    else if (op.equals("*"))                  // process the "*" operation
	    {
		result = (n1 * n2) + "";
		if (result.charAt(result.length()-2) == '.' &&
		    result.charAt(result.length()-1) == '0')
		    result = result.substring(0, result.length()-2);
		tf.setText(result);
	    }
	    else if (op.equals("/"))                  // process the "/" operation
	    {
		result = (n1 / n2) + "";
		if (result.charAt(result.length()-2) == '.' &&
		    result.charAt(result.length()-1) == '0')
		    result = result.substring(0, result.length()-2);
		tf.setText(result);
	    }
	}
	else                                        // process the digit and "."
	{                                           // buttons
	    String dig = ((JButton) e.getSource()).getText(); // get digit
	    String num = tf.getText();                // get text in text field 
	    if (num.equals("0"))                      // append the pressed
		num = dig;                              // digit to the number in
	    else                                      // text field
		num += dig;
	    tf.setText(num);
	}
    }    
}

class DrawPanel extends JPanel			    // main window panel
{
    @Override
    public void paintComponent(Graphics g)
    {
	super.paintComponent(g);		    // must be the 1st line
	// add here more code for drawing on panel 
    }
}