retain specific elements in an array from an array that are in another array

Question

let's say I have two array;

Integer[] array= { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 200, 5, 6, 5 };
Integer[] array2= { 12, 2 ,3, 2, 200, 5 };

Im trying to make a method that return an array with all the element removed except those who are in also present in array2, so the output of this method should be

{2 ,3, 5, 200, 5, 5 }

I dont want to use another data structure and i have no idea how to code what im trying to do, im not sure how i can determinate the resulting array length

thanks

Answer 1

If I understand your question, you could begin by creating a contains(Integer[], Integer) method. Iterate the array and return true if the array contains the value.

private static boolean contains(Integer[] a, Integer v) {
    for (Integer t : a) {
        if (t.equals(v)) {
            return true;
        }
    }
    return false;
}

Then you can leverage that to iterate your arrays twice. Once to perform a count, and the second time to populate a newly created array with the count number of elements. Something like,

public static Integer[] retainAll(Integer[] a, Integer[] b) {
    int count = 0;
    for (Integer val : a) {
        if (contains(b, val)) {
            count++;
        }
    }
    Integer[] out = new Integer[count];
    count = 0;
    for (Integer val : a) {
        if (contains(b, val)) {
            out[count++] = val;
        }
    }
    return out;
}

Then to test it,

public static void main(String[] args) {
    Integer[] array = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 200, 5, 6, 5 };
    Integer[] array2 = { 12, 2, 3, 2, 200, 5 };
    System.out.println(Arrays.toString(retainAll(array, array2)));
}

Output is the requested

[2, 3, 5, 200, 5, 5]

Of course, you could also use Arrays.asList(T...) and retainAll() like

public static Integer[] retainAll(Integer[] a, Integer[] b) {
    List<Integer> al = new ArrayList<>(Arrays.asList(a));
    al.retainAll(Arrays.asList(b));
    return al.toArray(new Integer[al.size()]);
}