REAL CALCULATOR IN JAVA GUI CODE

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package javaapplication29;



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

// A panel with calculator buttons and a result display

public class CalculatorPanel extends JPanel {

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

    public static void main(String[] args) {
        CalculatorPanel cp = new CalculatorPanel();
     
    }
    public JFrame frame ;
public CalculatorPanel(){
setLayout(new BorderLayout());
result = 0;
lastCommand = "=";
start = true;

// add the display
display = new JButton("0");
display.setEnabled(false);
add(display, BorderLayout.NORTH);

ActionListener insert = new InsertAction();
ActionListener command = new CommandAction();
frame = new JFrame("calculator");
frame.add(this);
frame.setVisible(true);
//add the buttons in a 4 x 4 grid
panel = new JPanel();
panel.setLayout(new GridLayout(4, 4));
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

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);

}

/**
* Adds a button to the center panel.
* @param label the button label.
* @param listener the button listener
*/

private void addButton(String label, ActionListener listener)
{
JButton button = new JButton(label);
button.addActionListener(listener);
panel.add(button);
}

/**
* This action insert the button action string to the end of the display text
*/

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);
}
}

/**
* This action executes the command that the button action string denotes
*/

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;
}
}
}

/**
* Carries out the pending calculation.
* @param X the value to be accumalated with the prior result
*/

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);
}
}