Java Collectors.groupingBy()---is List ordered? - Stack Overflow

Java Collectors.groupingBy()---is List ordered? - Stack Overflow

The documentation for groupingBy() says:

Implementation Requirements:

This produces a result similar to:

groupingBy(classifier, toList());

The documentation for toList() says:

Returns:

a Collector which collects all the input elements into a List, in encounter order

So, to answer your question, as long as your stream has a defined encounter order, you're guaranteed to get ordered lists.

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EDIT: As @Holger points out, groupingBy() would also have to respect encounter order to preserve toList()'s ordering constraint. The fact that it does is strongly implied in this note:

Implementation Note:

...If preservation of the order in which elements are presented to the downstream collector is not required, using groupingByConcurrent(Function, Collector) may offer better parallel performance.

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