Java Streams 0% Performance on Codility

Question

Regarding Codility's lesson OddOccurrencesInArray

I got total of 66% (Performance 0%) using the following code:

public int solution(int[] A) {
    return Arrays
            .stream(A)
            .parallel()
            .boxed()
            .collect(Collectors.groupingBy(n -> n, Collectors.counting()))
            .entrySet()
            .parallelStream()
            .filter(entry -> entry.getValue() % 2 != 0)
            .findAny()
            .map(Map.Entry::getKey)
            .orElseThrow();
}

However, I got total of 100% (Performance 100%) using code from here:

public int solution(int[] A) {
    HashMap<Integer, Integer> histogram = new HashMap<>();
    for (int i = 0; i < A.length; i++) {
        if (histogram.containsKey(A[i])) {
            int occurrences = histogram.get(A[i]);
            occurrences++;
            histogram.put(A[i], occurrences);
        } else {
            histogram.put(A[i], 1);
        }
    }
    Set<Integer> keySet = histogram.keySet();
    for (int currentKey : keySet) {
        int occurrences = histogram.get(currentKey);
        if (occurrences % 2 != 0) return currentKey;
    }
    throw new RuntimeException();
}

The following hybrid solution got total of 88% (1 Performace test failed):

public int solution(int[] A) {
    HashMap<Integer, Integer> histogram = new HashMap<>();
    for (int i = 0; i < A.length; i++) {
        if (histogram.containsKey(A[i])) {
            int occurrences = histogram.get(A[i]);
            occurrences++;
            histogram.put(A[i], occurrences);
        } else {
            histogram.put(A[i], 1);
        }
    }
    return histogram.entrySet()
            .parallelStream()
            .filter(n -> (n.getValue() % 2 != 0))
            .findAny()
            .map(Map.Entry::getKey)
            .orElseThrow();
}

I also tried to change all parallelStream() to stream(). Same results!

Are streams less efficient? or it's just Codility?

Answer 1

Codility fixed it, and all solutions are getting total of 100% (Performance 100%)