Java8 streams sequential and parallel execution produce different results?

Question

Running the following stream example in Java8:

    System.out.println(Stream
        .of(\"a\", \"b\", \"c\", \"d\", \"e\", \"f\")
        .reduce(\"\", (s1,         


        

Answer 1

From reduce's documentation:

The identity value must be an identity for the accumulator function. This means that for all t, accumulator.apply(identity, t) is equal to t.

Which is not true in your case - "" and "a" creates "/a".

I have extracted the accumulator function and added a printout to show what happens:

BinaryOperator accumulator = (s1, s2) -> {
    System.out.println("joining \"" + s1 + "\" and \"" + s2 + "\"");
    return s1 + "/" + s2;
};
System.out.println(Stream
                .of("a", "b", "c", "d", "e", "f")
                .parallel()
                .reduce("", accumulator)
);

This is example output (it differs between runs):

joining "" and "d"
joining "" and "f"
joining "" and "b"
joining "" and "a"
joining "" and "c"
joining "" and "e"
joining "/b" and "/c"
joining "/e" and "/f"
joining "/a" and "/b//c"
joining "/d" and "/e//f"
joining "/a//b//c" and "/d//e//f"
/a//b//c//d//e//f

You can add an if statement to your function to handle empty string separately:

System.out.println(Stream
        .of("a", "b", "c", "d", "e", "f")
        .parallel()
        .reduce((s1, s2) -> s1.isEmpty()? s2 : s1 + "/" + s2)
);

As Marko Topolnik noticed, checking s2 is not required as accumulator doesn't have to be commutative function.