search for example of inconsistent behavior java-8 stream?

Question

In java 8 documentation (doc order stream), one can see this :

if [a stream] is not ordered, repeated execution might produce different results.

Answer 1

The obvious answer is that whenever you use unordered you should get different results. For example using this:

int first = Arrays.asList(1, 2, 3, 4).stream()
           .unordered()
           .parallel()
           .findFirst()
           .get();
System.out.println(first);

should produce a result that is not always 1. Because the stream is unordered, so any result out of [1,2,3,4] is possible.

In java-8 this is not true, the stream pipeline does not take that unordered into account:

    @Override
    public  O evaluateParallel(PipelineHelper helper,
                                     Spliterator spliterator) {
        return new FindTask<>(this, helper, spliterator).invoke();
    }

But things have change in java-9:

    @Override
    public  O evaluateParallel(PipelineHelper helper,
                                     Spliterator spliterator) {
        // This takes into account the upstream ops flags and the terminal
        // op flags and therefore takes into account findFirst or findAny
        boolean mustFindFirst = StreamOpFlag.ORDERED.isKnown(helper.getStreamAndOpFlags());
        return new FindTask<>(this, mustFindFirst, helper, spliterator).invoke();
    }

So running the same code under java-9 multiple times will produce a different result.

There are operations that are already unordered like Stream#generate and Stream#forEach.