Notes

package calculator;

/**
*
* @author savchenko
*/
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;
}