Detect duplicated groups in stream

Question

I want to ensure that all numbers in the list are grouped together. Let me explain this on examples:

{1, 1, 1, 2, 2}    // OK, two distinct groups
{1, 1, 2, 2, 1         


        

Answer 1

In my opinion this problem doesn't fit the Stream API at all but I was curious how this could be implemenented (however in a performant way).

The problem is that you have to keep track of seen elements and the whole test should have a short-circuit behaviour. So I came up with this solution (without Streams):

public static boolean hasUniqueGroups(int[] arr) {
    Objects.requireNonNull(arr);
    Set seen = new HashSet<>();
    for (int i = 0; i < arr.length; i++) {
        if (i == 0 || arr[i] != arr[i - 1]) {
            if (!seen.add(arr[i])) {
                return false;
            }
        }
    }
    return true;
}

The next step is to introduce the Stream API and the solution looks like this:

public static boolean hasUniqueGroups(int[] arr) {
    Objects.requireNonNull(arr);
    Set seen = new HashSet<>();
    return IntStream.range(0, arr.length)
            .filter(i -> i == 0 || arr[i] != arr[i - 1])
            .mapToObj(i -> arr[i])
            .allMatch(seen::add);
}

Note: In order to parallelize this Stream you should use a thread-safe Set.