/*  Exercise 12.1 from "Java Certification Exam Guide" by Barry Boone p 256
    Following is written by extending the Thread class
    Re-write using 'implement Runnable'
    
    The applet continually displays numbers in blue, one number per second.
    If the number is a prime, the number is displayed in red.
 */
 
 import java.awt.*;
 import java.applet.Applet;
 
 public class Ex1 extends Applet {
    Color       color = Color.red;
    int         candidate = 3;
    PrimeThread prime;
    
    public void init() {
        prime = new PrimeThread(this);
        prime.start();
    }
    
    public void paint(Graphics g) {
        g.setColor(color);
        g.drawString(new Integer(candidate).toString(), 30, 40);        
    }            
 }
 
 class PrimeThread extends Thread {
    Ex1 target;
    
    PrimeThread(Ex1 target) {
        this.target = target;
    }
    
    public void run() {
        int candidate;
        for(candidate =3;;candidate++) {
            if(isPrime(candidate))
                target.color = Color.red;
            else
                target.color = Color.blue;
                
            target.candidate = candidate;
            target.repaint();
            
            try{
                sleep(1000);
            }catch(InterruptedException e) {}                
        } // end for              
    }
    
    public boolean isPrime(int number) {
        boolean isPrime = true;
        
        for(int i=2; i<number-1 && isPrime; i++) {
            if( (number%i) == 0 )
                isPrime = false;
        }
        return isPrime;
    }
 }