How would I go about using multiple arraylists to filter factors of a number?

Okay, so say i want to find factors of a number a. I know how to do that, using a for loop and an arraylist. My problem is that (and i have no idea how to go about this) I want the factors in pairs.(that multiply together to get the original number) I would assume I could get the factors in multiple arraylists that each have 2 variables.. but that is extremely unwieldy as different numbers will have different numbers of factors. (Not to mention, I dont know how to do this..)

After this, I want to test those variables in an equation to narrow them down. Say, whichever of those pairs add up to a given number is the one I want to use. I have been trying to figure out how to go about all of this using tutorials but I am completely lost, as I am somewhat new to Java.

Thank you for any and all help :-)

Solution

You could create your own class that had fields for each side of the pair, but it would be acceptable and convenient to use a Map<Integer, Integer>. A basic implementation would be:

private static Map<Integer, Integer> factorize(int number) {
    Map<Integer, Integer> factors = new HashMap<Integer, Integer>();
    for (int i = 1; i < Math.sqrt(number); i++) {
        if (number % i == 0)
            factors.put(i, number / i);
    }
    return factors;
}

Here's some test code to show how it would be called and what the result is:

public static void main(String[] args) {
    Map<Integer, Integer> factorPairs = factorize(12345);
    for (Map.Entry<Integer, Integer> factor : factorPairs.entrySet())
        System.out.println(factor.getKey() + " x " + factor.getValue());
}

Output:

That should be enough to get you started