What is the best way to build a stream iteratively?

Question

Suppose we have an algorithm that generates items sequentially in a loop (one item per iteration) and we want to put this algorithm into a method that returns a stream of those items.

Which approach is best; this one?

Stream<Cell> streamCellsTouchingRay(Point2D fromPoint, Vector2D direction){ // ... Stream<Cell> stream = Stream.of(/* 1st item */); while(/* ... */){ // ... stream = Stream.concat(stream, /* i'th item */); } }

...or this one?

Stream<Cell> streamCellsTouchingRay(Point2D fromPoint, Vector2D direction){ // ... ArrayList<Cell> cells = new ArrayList<>(/* required capacity is known */); cells.add(/* 1st item */); while(/* ... */){ // ... cells.add(/* i'th item */); } return cells.stream(); }

...or another approach entirely?

Why don't you simple return a list if you're holding all items in memory instead of generating them on demand? — xehpuk
– xehpuk, Commented Sep 10, 2015 at 19:42

Louis Wasserman · Accepted Answer · 2015-09-10 18:14:23Z

Another approach entirely: use Stream.Builder.

Stream<Cell> streamCellsTouchingRay(Point2D fromPoint, Vector2D direction){ // ... Stream.Builder<Cell> cells = Stream.builder(); cells.add(/* 1st item */); while(/* ... */){ // ... cells.add(/* i'th item */); } return cells.build(); }

Community · Accepted Answer · 2017-05-23 12:14:40Z

Using an ArrayList will have a some copying overhead when you finally get the stream.
When using Stream.concat(), the docs note that you should use caution when concatenating many items:

Use caution when constructing streams from repeated concatenation. Accessing an element of a deeply concatenated stream can result in deep call chains, or even StackOverflowException.

Louis Wasserman's asnwer seems to be the best, if you are concatenating many items.

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