Java array, add item into next empty index

Question

In Java, is there a way to add a value not at a specific index, but to the next undeclared index? Say:

int[] negativeArray = new int[21];
int[] positiveArray = new int[21];

There are two arrays for two different types of ints, for example negative and positive. I'm looping through this, and I want it to work something like a stack (I don't know too much about stacks, but to my knowledge you don't go through its index, you just pop/push items onto it) to where if its a negative number, put in the number in the next undeclared index spot in the negative array.

I thought of a way to do this with some extra code. I'd set all values in the array to 0. When checking if the variable is negative or positive, I would loop through the array to the next value that is 0. Once I find it I know what index I'm on. This would require a bit of effort though, are there any simpler ways of doing this?

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Edit: Some comments are stating different ways to do this without using a basic array. I was assigned this, and I am required to use an array to get credit for it...

Answer 1

If you are actually using the array as a stack (and thus, will only add or remove items at the top of the stack) then you could keep in another variable the next free index in the array.

int[] array = new int[21];
int nextIndex = 0;

public void push(int e) {
    array[nextIndex] = e;
    ++nextIndex;
}

public int pop() {
    --nextIndex;
    return array[nextIndex];
}

If removals can occur anywhere, then I don't see a better solution than iterating over the array to find a free spot.

Answer 2

That is why Listhave been made. Simply use something like this:

List<Integer> negativeIntegers = new ArrayList<Integer>(21);
...
negativeIntegers.add(-127);

Answer 3

If you want to create an array with the positive values and one with the negative values, you can do it with the algorithm you propose:

public static void main(String[] args) throws Exception {
    int[] negativeArray = new int[3];
    int[] positiveArray = new int[3];

    int[] test = new int[] {1, -1, 2, -2, 3, -3};

    int posIndex = 0;
    int negIndex = 0;

    for (int i = 0; i < test.length; i++) {
        if (test[i] > 0) {
            positiveArray[posIndex++] = test[i];
        } else if (test[i] < 0) {
            negativeArray[negIndex++] = test[i];
        }
    }

    System.out.println(Arrays.toString(test)); //[1, -1, 2, -2, 3, -3]
    System.out.println(Arrays.toString(positiveArray)); //[1, 2, 3]
    System.out.println(Arrays.toString(negativeArray)); //[-1, -2, -3]
}