What is the order in which stream operations are applied to list elements? [duplicate]

Answer 1

Are there any guarantees in the JLS regarding the order in which stream operations are applied to the list elements?

Quoting from Ordering section in Stream javadocs

• Streams may or may not have a defined encounter order. Whether or not a stream has an encounter order depends on the source and the intermediate operations.

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Applying the filter predicate to "bc" is not going to happen before applying the filter predicate to "a"?

As quoted above, streams may or may not have a defined order. But in your example since it is a List, the same Ordering section in Stream javadocs goes on saying that

• If a stream is ordered, most operations are constrained to operate on the elements in their encounter order; if the source of a stream is a List containing [1, 2, 3], then the result of executing map(x -> x*2) must be [2, 4, 6].

  Applying the above statement to your example - I believe, the filter predicate would receive the elements in order defined in the List.

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Or, applying the mapping function to "def" is not going to happen before applying the mapping function to "a"?

For this I would refer to the Stream operations section Stream operations in Streams, that says,

• Stateless operations, such as filter and map, retain no state from previously seen element when processing a new element

  Since map() doesn't retain state, I believe, it is safe to assume "def" is not going to be processed before "a" in your example.

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1 will be printed before 3?

Although it may be unlikely with sequential streams (like List) but not guaranteed as the Ordering section in Stream javadocs does indicate that

• some terminal operations may ignore encounter order, such as forEach().