Hello.java

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

public class Hello extends JFrame implements ActionListener   {		
	private JPanel panel;
	private JTextField display;
	private JButton[] buttons;
	private String[] labels = { "Backspace", "", "", "CE", "C", "7", "8", "9",
			"/", "sqrt", "4", "5", "6", "x", "%", "1", "2", "3", "-", "1/x",
			"0", "-/+", ".", "+", "=", };
	private double result = 0;
	private String operator = "=";
	private boolean startOfNumber = true;
	
	public Hello() {
		display = new JTextField(35);
		panel = new JPanel();
		display.setText("0.0");
		display.setEnabled(true);
		
		panel.setLayout(new GridLayout(0, 5, 3, 3));
		buttons = new JButton[25];
		int index = 0;
		
		for (int rows = 0; rows < 5; rows++) 
		{
			for (int cols = 0; cols < 5; cols++)
			{
				buttons[index] = new JButton(labels[index]);
				if (cols >= 3)
					buttons[index].setForeground(Color.red);
				else
					buttons[index].setForeground(Color.blue);
				buttons[index].setBackground(Color.yellow);
				panel.add(buttons[index]);
				buttons[index].addActionListener(this);
				index++;
			}
		}
		add(display, BorderLayout.NORTH);
		add(panel, BorderLayout.CENTER);
		setVisible(true);
		pack();
	}
	
	public void actionPerformed(ActionEvent e)
	{
		String command = e.getActionCommand();
		if (command.charAt(0) == 'C')
		{
			startOfNumber = true;
			result = 0;
			operator = "=";
			display.setText("0.0");
		} 
		
		else if (command.charAt(0) >= '0' && command.charAt(0) <= '9' || command.equals("."))
		{
			if (startOfNumber == true)
				display.setText(command);
			else
				display.setText(display.getText() + command);
			startOfNumber = false;
		}
		
		else 
		{
			if (startOfNumber)
			{
				if (command.equals("-"))
				{
					display.setText(command);
					startOfNumber = false;
				}
				else
					operator = command;
			}
			
			else 
			{
				double x = Double.parseDouble(display.getText());
				calculate(x);
				operator = command;
				startOfNumber = true;
			}
		}
	}
	
	private void calculate(double n) 
	{
		if (operator.equals("+"))
			result += n;
		else if (operator.equals("-"))
			result -= n;
		else if (operator.equals("x"))
			result *= n;
		else if (operator.equals("/"))
			result /= n;
		else if (operator.equals("="))
			result = n;
		else if(operator.equals("sqrt"))
			result = Math.sqrt(n);
		else if(operator.equals("1/x"))
			
		display.setText("" + result);
	}
	
	public static void main(String[] args) 
	{
		Hello s = new Hello();
	}		
}