Labelled "continue" doesn't seem to work

Question

Just started learning java and I can't understand what's wrong with my code. PrimeIterator is supposed to generate infinite amount of prime numbers (starting from a number 3) but when I print the output I get: 3, 5, 7, 9, 11, 13, 15 etc.

public class Prime {

    PrimeIterator iter = new PrimeIterator();

    private class PrimeIterator implements java.util.Iterator<Integer>
    {
        int numb = 1;

        public boolean hasNext() 
        {
            return true;
        }

        public Integer next() 
        {
            nextCandidate:
            do{
                numb += 2;
                int numbSqrt = (int)java.lang.Math.sqrt(numb);

                for (int i = 3; i <= numbSqrt; i = i+2)
                {
                    if (numb % i == 0)
                    {
                        continue nextCandidate;
                    }
                }
            }while(false);
            return numb;
        }

        public void remove() {}
    }

    void printPrimes()
    {
        System.out.print(2);
        while(iter.hasNext())
        {
            try 
            {
                Thread.sleep(500);
            } catch (InterruptedException e) 
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            System.out.print(", " + iter.next());   
        }
    }
}

I wanted to use labelled "continue" statement for my do-while loop. However my intuition tells me that I use it incorrectly.

Answer 1

The trouble is with the while(false) continuation condition. Being a do while(false) statement, this means that it can never loop more than once. That is, when you try to jump the execution to the labeled statement, it won't cycle again through the do while because the continuation condition (false) is not validated, even if you think that continue will make the execution loop again.

So it will never increment numb more than once per next() method execution.

I'd do something like the following:

nextCandidate:
do{
    numb += 2;
    int numbSqrt = (int)java.lang.Math.sqrt(numb);

    for (int i = 3; i <= numbSqrt; i = i+2)
    {
        if (numb % i == 0)
        {
            continue nextCandidate;
        }
    }
    break;

}while(true);

Answer 2

Here are the problems that I see

1. You're not even printing output.

2. (int)java.lang.Math.sqrt(5) will end up truncating to 2. You should add 1 to your square root, since it's a problem if you don't iterate enough, but it's not a problem if you iterate more than you need to.

3. when you do find a prime number, your for loop will end, and while(false) will terminate the do-while loop