grzeg_poreb.java

 /* Optimized for not checking multiples of 3 */
    private static boolean isPrime(double val) {
        if (val == 0 || val == 1) return false;
        if (val == 2 || val == 3) return true;
        if (val % 2 == 0 || val % 3 == 0) return false;

        for (int i = 5; i <= Math.sqrt(val); i += 4) {
            if (val % i == 0) return false;
            i += 2;
            if (val % i == 0) return false;
        }
        return true;
    }

    private static Integer[] subtractPrimeOccurrences(int []originalArray, int[] subtractorArray){
        /* <number, occurrences> */
        HashMap<Integer, Integer> subtractorArrayOccurrences = new HashMap<>();
        List<Integer> result = new ArrayList<>();

        for (int nr : subtractorArray) 
            subtractorArrayOccurrences.compute(nr, (key, val) -> val == null ? 1 : val + 1);
        
        for (int nr : originalArray) {
            int nrOfOcc = subtractorArrayOccurrences.getOrDefault(nr, 0);

            if (!isPrime(nrOfOcc))
                result.add(nr);
        }

        return result.toArray(new Integer[0]);
    }