takeWhile() working differently with flatmap

问题:

I am creating snippets with takeWhile to explore its possibilities. When used in conjunction with flatMap, the behaviour is not in line with the expectation. Please find the code snippet below.

String[][] strArray = {{"Sample1", "Sample2"}, {"Sample3", "Sample4", "Sample5"}};

Arrays.stream(strArray)
        .flatMap(indStream -> Arrays.stream(indStream))
        .takeWhile(ele -> !ele.equalsIgnoreCase("Sample4"))
        .forEach(ele -> System.out.println(ele));

Actual Output:

Sample1
Sample2
Sample3
Sample5

ExpectedOutput:

Sample1
Sample2
Sample3

Reason for the expectation is that takeWhile should be executing till the time the condition inside turns true. I have also added printout statements inside flatmap for debugging. The streams are returned just twice which is inline with the expectation.

However, this works just fine without flatmap in the chain.

String[] strArraySingle = {"Sample3", "Sample4", "Sample5"};
Arrays.stream(strArraySingle)
        .takeWhile(ele -> !ele.equalsIgnoreCase("Sample4"))
        .forEach(ele -> System.out.println(ele));

Actual Output:

Sample3

Here the actual output matches with the expected output.

Disclaimer: These snippets are just for code practise and does not serve any valid usecases.

Update: Bug JDK-8193856: fix will be available as part of JDK 10. The change will be to correct whileOps Sink::accept

@Override 
public void accept(T t) {
    if (take = predicate.test(t)) {
        downstream.accept(t);
    }
}

Changed Implementation:

@Override
public void accept(T t) {
    if (take && (take = predicate.test(t))) {
        downstream.accept(t);
    }
}

回答1:

This is a bug in JDK 9 - from issue #8193856:

takeWhile is incorrectly assuming that an upstream operation supports and honors cancellation, which unfortunately is not the case for flatMap.

Explanation

If the stream is ordered, takeWhile should show the expected behavior. This is not entirely the case in your code because you use forEach, which waives order. If you care about it, which you do in this example, you should use forEachOrdered instead. Funny thing: That doesn't change anything.