In other words, what hobovsky is saying is that you need to make a copy of input array.

In Java , you're not allowed to modify the input Array/list/Vector

It's a test against mutation of input. It does not matter what the input is, your solution fails it because it probably modifies the input array.

How many times do each of those numbers that 2 is a factor of occur?

The case 5,0 was only an example...

a = [15, 30, -45]
2 is a factor of 30 the sum of numbers divisible by 2 is 30 hence a partial result [2, 30]
3 is a factor of 15, 30, -45 the sum of numbers divisible by 3 is 0 hence a partial result [3, 0]
5 is a factor of 15, 30, -45 the sum of numbers divisible by 5 is 0 hence a partial result [5, 0]
So the final result is: [[2, 30], [3, 0], [5, 0]]