Java8流顺序和并行执行产生不同的结果？

Question

在Java8中运行以下流示例：

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

产量：

/a/b/c/d/e/f

当然，这并不奇怪。由于http://docs.oracle.com/javase/8/docs/api/index.html?overview-summary.html，流是顺序执行还是并行执行并不重要：

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

/a//b//c//d//e//f

顺便说一下：使用（首选的）.collectors.collectors.joining（“/”）)而不是reduce(...)a/b/c/d/e/f生成相同的结果，用于顺序和并行执行。

JVM详细信息：

java.specification.version: 1.8
java.version: 1.8.0_31
java.vm.version: 25.31-b07
java.runtime.version: 1.8.0_31-b13

Answer 1

摘自Reduce的文档：

标识值必须是累加器函数的标识。这意味着对于所有t，accumulator.apply(identity，t)等于t。

这在您的情况下是不正确的-“”和“a”创建“/a”。

BinaryOperator<String> 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)
);

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

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