What is the difference between .foreach and .stream().foreach? [duplicate]

Question

This is a example: code A:

files.forEach(f -> {
    //TODO
});

and another code B may use on this way:

files.stream().forEach(f -> { });

What is the difference between both, with stream() and no stream()?

Answer 1

Practically speaking, they are mostly the same, but there is a small semantic difference.

Code A is defined by Iterable.forEach, whereas code B is defined by Stream.forEach. The definition of Stream.forEach allows for the elements to be processed in any order -- even for sequential streams. (For parallel streams, Stream.forEach will very likely process elements out-of-order.)

Iterable.forEach gets an Iterator from the source and calls forEachRemaining() on it. As far as I can see, all current (JDK 8) implementations of Stream.forEach on the collections classes will create a Spliterator built from one of the source's Iterators, and will then call forEachRemaining on that Iterator -- just like Iterable.forEach does. So they do the same thing, though the streams version has some extra setup overhead.

However, in the future, it's possible that the streams implementation could change so that this is no longer the case.

(If you want to guarantee ordering of processing streams elements, use forEachOrdered() instead.)

Answer 2

There is no difference in terms of semantics, though the direct implementation without stream is probably slightly more efficient.