Return all prime numbers smaller than M

Question

Given an integer M. return all prime numbers smaller than M.

Give a algorithm as good as you can. Need to consider time and space complexity.

Answer 1

This is what I developed for Seive of Eratosthenes. There would be better implementations,of course.

//finds number of prime numbers less than length

private static int findNumberOfPrimes(int length) {
    int numberOfPrimes = 1;
    if (length == 2) {
        return 1;
    }

    int[] arr = new int[length];
    //creating an array of numbers less than 'length'
    for (int i = 0; i < arr.length; i++) {
        arr[i] = i + 1;
    }
    //starting with first prime number 2, all the numbers divisible by 2(and upcoming) is replaced with -1
    for (int i = 2; i < arr.length && arr[i] != -1; i++) {

        for (int j = i; j < arr.length; j++) {
            if (arr[j] % arr[i] == 0) {
                arr[j] = -1;
                numberOfPrimes += 1;
            }
        }

    }
    return numberOfPrimes;
}