Difference between iterable.forEach() and iterable.stream().forEach()

Question

It looks like I can call list.forEach(a -> a.stuff()) directly on my collection, instead of list.stream().forEach(a -> a.stuff()). When would I use one over the other (parallelStream() aside..)?

Answer 1

There are a few differences:

Iterable.forEach guarantees processing in iteration order, if it's defined for the Iterable. (Iteration order is generally well-defined for Lists.) Stream.forEach does not; one must use Stream.forEachOrdered instead.

Iterable.forEach may permit side effects on the underlying data structure. Although many collections' iterators will throw ConcurrentModificationException if the collection is modified during iteration, some collections' iterators explicitly permit it. See CopyOnWriteArrayList, for example. By contrast, stream operations in general must not interfere with the stream source.

If the Iterable is a synchronized wrapper collection, for example, from Collections.synchronizedList(), a call to forEach on it will hold its lock during the entire iteration. This will prevent other threads from modifying the collection during the iteration, ensuring that the iteration sees a consistent view of the collection, and preventing ConcurrentModificationException. (This will also prevent other threads from reading the collection during the iteration.) This is not the case for streams. There is nothing to prevent the collection from being modified during the stream operation, and if modification does occur, the result is undefined.