Work out the prime factors from a given integer and also the number of factors for that integer. Adopted from http://introcs.cs.princeton.edu/java/13flow/Factors.java.html

Factors.java

import java.util.*;

public class Factors {

    public static List<Long> primeFactors(long n) {
        List<Long> result = new ArrayList<Long>();

        // for each potential factor i
        for (long i = 2; i*i <= n; i++) {
            // if i is a factor of N, repeatedly divide it out
            while (n % i == 0) {
                result.add(i);
                n = n / i;
            }
        }

        // if biggest factor occurs only once, n > 1
        if (n > 1) result.add(n);

        return result;
    }

    public static int numberOfFactors(long n) {
        List<Long> primeFactors = primeFactors(n);
        HashSet<Long> uniqueFactors = new HashSet<Long>(primeFactors);

        int sum = 1;
        for (long l : uniqueFactors) {
            int occurs = Collections.frequency(primeFactors, l);
            sum *= occurs + 1;
        }

        return sum;
    }
}