java - Java 8 Stream 中的 forEach 与 forEachOrdered

Question

我了解这些方法的执行顺序不同，但在我的所有测试中，我无法实现不同的执行顺序。

例子:

System.out.println("forEach Demo");
Stream.of("AAA","BBB","CCC").forEach(s->System.out.println("Output:"+s));
System.out.println("forEachOrdered Demo");
Stream.of("AAA","BBB","CCC").forEachOrdered(s->System.out.println("Output:"+s));

输出:

forEach Demo
Output:AAA
Output:BBB
Output:CCC
forEachOrdered Demo
Output:AAA
Output:BBB
Output:CCC

请提供两种方法产生不同输出的示例。

Answer 1

Stream.of("AAA","BBB","CCC").parallel().forEach(s->System.out.println("Output:"+s));
Stream.of("AAA","BBB","CCC").parallel().forEachOrdered(s->System.out.println("Output:"+s));

第二行会一直输出

Output:AAA
Output:BBB
Output:CCC

而第一个没有保证，因为订单没有保留。 forEachOrdered 将按照其源指定的顺序处理流的元素，而不管流是顺序的还是并行的。

引自 forEach Javadoc:

The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does not guarantee to respect the encounter order of the stream, as doing so would sacrifice the benefit of parallelism.

当forEachOrdered Javadoc 状态(强调我的):

Performs an action for each element of this stream, in the encounter order of the stream if the stream has a defined encounter order.