Generating primes. i know the problem, just can't fix it.

Question

I have to generate all the primes up to a given integer, but I'm failing horribly. I have to stay pretty close to what I've coded so far.
Here's the class I constructed
public class PrimeGenerator
     public PrimeGenerator()
     num=0;
     public PrimeGenerator(int num2)
     num=num2;
     public boolean isPrime()
     if (num==1)
          return false;
     if (num==2)
          return true;
     if ((num==3) || (num==5) || (num==7))
          return true;
     else if ((num%2==0) && (num%3==0) && (num%5==0) && (num%7==0))
          return false;
     return true;
     public int nextPrime()
     num++;
     while (!(isPrime()))
          num++;
     return num;
     private int num;
}Here's the test program I wrote to test it
import javax.swing.JOptionPane;
public class PrimeGeneratorTest
     public static void main(String[] args)
     String input=JOptionPane.showInputDialog("Enter a number");
     int limit = Integer.parseInt(input);
     PrimeGenerator prime=new PrimeGenerator(limit);
     int p = prime.nextPrime();
     while (limit>p)
          System.out.print(p + " ");
          p =prime.nextPrime();     
     System.exit(0);
}The only problem (I hope) is that the integer num of class PrimeGenerator is going to be set to that same as value as the object limit of the test, and the while loop in the test program is never executed. But everytime I change something in either program the result is just a different wrong answer. For example if I set num=0; in nextPrime(), then the while loop will execute in the program and it will be an endless loop displaying the number 2.
Actually its probably easier to look at the program than listen to me...but I really need to figure out how to correct this because its eating away at my soul.

Answer 1

Your main problem is, that your algorithm, that detects, wheter num is a prime number or not is wrong. Try to use another one (like the "Sieve of Eratosthenes"). Your PrimeGenerator class could then look something like this:
import java.util.ArrayList;
import java.util.Iterator;
public class PrimeGenerator{
  private int num;
  private ArrayList a;
  public PrimeGenerator() {
    num=1;
    a=new ArrayList();
  public boolean isPrime()  {
    Iterator iter = a.iterator();
    int i;
    while(iter.hasNext()) {
      i = ((Integer)iter.next()).intValue();
      if(num%i == 0) {
        return false;
    a.add(new Integer(num));
    return true;
  public int nextPrime()  {
    num++;
    while (!(isPrime())) num++;
    return num;
}Plus there is another problem with your test-class:
  PrimeGenerator prime=new PrimeGenerator(limit);Why do you set the num-field of the PrimeGenerator class to the limit you do not want to exceed in the testclass?