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);
			}
		});
	}
}
class CalculatorFrame extends JFrame {
	public CalculatorFrame() {
		setTitle("Calculator");
		CalculatorPanel panel = new CalculatorPanel();
		add(panel);
		pack();
	}
}
class CalculatorPanel extends JPanel {
	public CalculatorPanel() {
		setLayout(new BorderLayout());

		result = 0;
		lastCommand = "=";
		start=true;

		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);
	}
	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);
		}
	}
	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;
			}
		}
	}
	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);
	}
	private JButton display;
	private JPanel panel;
	private double result;
	private String lastCommand;
	private boolean start;
}