Adding Functions To A Calculator

I've made a calculator that can add, subtract, multiply, and divide, but I still need a clear function and a square root one for it. I've tried going on the various calculator tutorials, but they don't work.

And here's my code:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class Calculator extends JPanel implements ActionListener
{
private JTextField display = new JTextField("0");
private String button = "789/456*123-0.+=";
private double result = 0;

[Code] .....