java - Very simple prime number test - I think I'm not understanding the for loop

Question

I am practicing past exam papers for a basic java exam, and I am finding it difficult to make a for loop work for testing whether a number is prime. I don't want to complicate it by adding efficiency measures for larger numbers, just something that would at least work for 2 digit numbers.

At the moment it always returns false even if n IS a prime number.

I think my problem is that I am getting something wrong with the for loop itself and where to put the "return true;" and "return false;"... I'm sure it's a really basic mistake I'm making...

public boolean isPrime(int n) {
    int i;
    for (i = 2; i <= n; i++) {
        if (n % i == 0) {
            return false;
        }
    }
    return true;
}

The reason I couldn't find help elsewhere on stackoverflow is because similar questions were asking for a more complicated implementation to have a more efficient way of doing it.

Answer 1

You can stop much earlier and skip through the loop faster with:

public boolean isPrime(long n) {
    // fast even test.
    if(n > 2 && (n & 1) == 0)
       return false;
    // only odd factors need to be tested up to n^0.5
    for(int i = 3; i * i <= n; i += 2)
        if (n % i == 0) 
            return false;
    return true;
}