java - Stream reduce vs Stream.parallel.reduce()

Question

这个问题在这里已经有了答案:





What do the Stream reduce() requirements exactly entail?

(4 个回答)


1年前关闭。




我试图理解为什么这个例子的结果总是如此，这是我的例子:

 String s1 = Arrays.asList("A", "E", "I", "O", "U").stream()
                .reduce("", String::concat);
 String s2 = Arrays.asList("A", "E", "I", "O", "U").parallelStream()
                .reduce("", String::concat);

System.out.println(s1.equals(s2));

这总是打印 true ，我所知道的是对于 parallelStream 我们无法预测结果有人可以解释为什么吗？

Answer 1

如果您查看 Stream.reduce() 的文档你会发现:

Performs a reduction on the elements of this stream [...] and returns the reduced value. This is equivalent to:

 T result = identity;
 for (T element : this stream)
     result = accumulator.apply(result, element)
 return result;

but is not constrained to execute sequentially.

所以Stream.reduce()确保按顺序处理值。

如果您尝试使用 Stream.forEach()并像这样打印每个值:

Arrays.asList("A", "E", "I", "O", "U").stream()
        .forEach(System.out::print);
System.out.println();
Arrays.asList("A", "E", "I", "O", "U").parallelStream()
        .forEach(System.out::print);

您将得到以下结果(或第二行中的类似结果):

AEIOU
IUOEA

( Stream.forEachOrdered() 上面的例子也将按顺序打印值)