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

public class Calc
{  
    public static void main(String[] args) 
    {
	SwingApp app = new SwingApp(250, 150);	    // set app window size
	app.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
    }
}

class SwingApp extends JFrame implements ActionListener
{
    private final DrawPanel panel;		    // class variable
    private final JTextField n1, n2, res;
    private final JButton plusB, minusB, timesB, overB, clearB;
  
    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.setBackground(Color.yellow);
	panel.setLayout(new GridLayout(3, 2, 10, 10));
	
	n1 = new JTextField(6);			    // text fields
	n2 = new JTextField();
	res = new JTextField();
 
	plusB = new JButton(" + ");		    // buttons 
	minusB = new JButton(" - ");
	timesB = new JButton(" x ");
	overB = new JButton(" / ");
	clearB = new JButton(" C ");	

	JPanel p1 = new JPanel();              // "left" panel
	p1.setLayout(new GridLayout(1, 2, 10, 0));
	p1.add(plusB);
	p1.add(minusB);
	
	JPanel p2 = new JPanel();              // "right" panel
	p2.setLayout(new GridLayout(1, 2, 10, 0));
	p2.add(timesB);
	p2.add(overB);

	panel.add(n1);                  
	panel.add(p1);
	panel.add(n2);
	panel.add(p2);
	panel.add(res);
	panel.add(clearB); 

	plusB.addActionListener(this);         // set up event listeners
	minusB.addActionListener(this);
	timesB.addActionListener(this);
	overB.addActionListener(this);
	clearB.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);	
	// add here more code for GUI components
    }
    
// this method is called if any button is pressed
    @Override
    public void actionPerformed(ActionEvent e)
    {
	String s = n1.getText();               // get the numbers from the
	double n1 = Double.parseDouble(s);     // GUI
	s = n2.getText();
	double n2 = Double.parseDouble(s);

	if (e.getSource() == plusB)            // ckeck which button was clicked 
	    res.setText((n1+n2)+"");             // compute the result and
	else if (e.getSource() == minusB)      // display it
	    res.setText((n1-n2)+"");
	else if (e.getSource() == timesB)
	    res.setText((n1*n2)+"");
	else if (e.getSource() == overB)
	    res.setText((n1/n2)+"");
	else if (e.getSource() == clearB)      // clear all input fields
	{
	    this.n1.setText("");
	    this.n2.setText("");
	    res.setText("");
	}
    }    
}

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 
    }
}