Why filter() after flatMap() is “not completely” lazy in Java streams?

Question

I have the following sample code:

System.out.println(
       \"Result: \" +
        Stream.of(1, 2, 3)
                .filter(i -> {
                             


        

Answer 1

In my free StreamEx library I introduced the short-circuiting collectors. When collecting sequential stream with short-circuiting collector (like MoreCollectors.first()) exactly one element is consumed from the source. Internally it's implemented in quite dirty way: using a custom exception to break the control flow. Using my library your sample could be rewritten in this way:

System.out.println(
        "Result: " +
                StreamEx.of(1, 2, 3)
                .flatMap(i -> Stream.of(i - 1, i, i + 1))
                .flatMap(i -> Stream.of(i - 1, i, i + 1))
                .filter(i -> {
                    System.out.println(i);
                    return true;
                })
                .collect(MoreCollectors.first())
                .get()
        );

The result is the following:

-1
Result: -1