public class PrimeRunner
{
    // Get a number
    // Display all the primes up to that number
}

public class PrimeGenerator
{
    public int NextPrime()
    {
        // Display the next prime number
    }
}

==========================

public class PrimeRunner
{
    System.out.print("Maximum? ");
    int max = in.nextInt();
    
    prime = pg.nextPrime()
    while (prime <= max)
    {
        System.out.println(prime)
        prime = pg.nextPrime()
    }
}

public class PrimeGenerator
{
    int currentNumber = 0

    public int NextPrime()
    {
        // Keep increasing currentNumber until it is prime
        return currentNumber
    }
}

==========================

// And so on...