Sunday, 6 August 2017

forEach vs forEachOrdered in Java 8 Stream

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));


The second line will always output



Output:AAA
Output:BBB
Output:CCC



whereas the first one is not guaranted since the order is not kept. forEachOrdered will processes the elements of the stream in the order specified by its source, regardless of whether the stream is sequential or parallel.



Quoting from 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.




When the forEachOrdered Javadoc states (emphasis mine):





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


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