Source code:

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class PiSimple extends Applet implements ActionListener {

TextField tf;
double piOverFour;
int attempts;
Label instructions, colorChange;

public void init()
{
    attempts = 0;
    instructions = new Label("Type number up to a billion and click enter");
    add(instructions);
    tf = new TextField(9);
    add(tf);
    tf.addActionListener(this);
    colorChange = new Label("Color will change when calculation finished");
    add(colorChange);
}

public void paint(Graphics g)
{
    g.drawString("Approximation of pi = " + String.valueOf(4.0*piOverFour), 10 , 100);
    g.drawString("Actual first dig of pi = 3.14159265358979323846264338....", 10 , 120);
}

static final int getIntFromString(String s)
{
    s = s.trim();
    if (s.equals("") || s == null) return (1);
    try
    {
    return (Integer.valueOf(s).intValue());
    }
    catch (Exception e)
    {
    return(1);
    }
}

public void actionPerformed (ActionEvent e)
{
    attempts++;
    int terms = getIntFromString(tf.getText());
    if (terms > 1000000000) terms = 1000000000;
    piOverFour = 0.0;
    for (int i=0; i<terms; i++)
    {// implements formula pi/4 = 1 -1/3 + 1/5 -1/7 + 1/9 .....
        if (i%2 == 0) piOverFour += 1.0/((double)(2*i + 1));
        else piOverFour -= 1.0/((double)(2*i + 1));
    }
    Color color = new Color(225, 225 - ((attempts%2)*30), 255);
    setBackground(color);
    instructions.setBackground(color);
    colorChange.setBackground(color);
    repaint();
}
} // END OF Class PiSimple