Java Program for calculator

import java.awt.BorderLayout;
import java.awt.Button;
import java.awt.Container;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
 
import javax.swing.JApplet;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
 
public class PopupCalculatorApplet extends JApplet implements ActionListener {
public void init() {
Button calcButton = new Button("Calculator");
calcButton.addActionListener(this);
Container contentPane = getContentPane();
contentPane.add(calcButton);
}
 
public void actionPerformed(ActionEvent evt) {
if (calc.isVisible())
calc.setVisible(false);
else
calc.show();
}
 
private JFrame calc = new CalculatorFrame();
}
 
class CalculatorPanel extends JPanel implements ActionListener {
public CalculatorPanel() {
setLayout(new BorderLayout());
 
display = new JTextField("0");
display.setEditable(false);
add(display, "North");
 
JPanel p = new JPanel();
p.setLayout(new GridLayout(4, 4));
String buttons = "789/456*123-0.=+";
for (int i = 0; i < buttons.length(); i++)
addButton(p, buttons.substring(i, i + 1));
add(p, "Center");
}
 
private void addButton(Container c, String s) {
JButton b = new JButton(s);
c.add(b);
b.addActionListener(this);
}
 
public void actionPerformed(ActionEvent evt) {
String s = evt.getActionCommand();
if ('0' <= s.charAt(0) && s.charAt(0) <= '9' || s.equals(".")) {
if (start)
display.setText(s);
else
display.setText(display.getText() + s);
start = false;
} else {
if (start) {
if (s.equals("-")) {
display.setText(s);
start = false;
} else
op = s;
} else {
calculate(Double.parseDouble(display.getText()));
op = s;
start = true;
}
}
}
 
public void calculate(double n) {
if (op.equals("+"))
arg += n;
else if (op.equals("-"))
arg -= n;
else if (op.equals("*"))
arg *= n;
else if (op.equals("/"))
arg /= n;
else if (op.equals("="))
arg = n;
display.setText("" + arg);
}
 
private JTextField display;
 
private double arg = 0;
 
private String op = "=";
 
private boolean start = true;
}
 
class CalculatorFrame extends JFrame {
public CalculatorFrame() {
setTitle("Calculator");
setSize(200, 200);
 
Container contentPane = getContentPane();
contentPane.add(new CalculatorPanel());
}
}