what does java8 stream map do here?

Question

I was confused about the difference between map() and forEach() method in java8 stream. For instance,

List strings =          


        

Answer 1

The difference is this:

• The idea of forEach() is to "work" on each element of the underlying collection (by having a side effect) whereas
• map() is about applying a method on each object and putting the result of that into a new stream

That is also the reason why your stream().map() doesn't result in something - because you throw away the new stream created by the map() call!

In that sense, the signatures of the two methods tell you that:

void forEach(BiConsumer action)

Performs the given action for each entry in this map until all entries have been processed

versus

  Stream map(Function mapper)

Returns a stream consisting of the results of applying the given function to the elements of this stream.

And for the record: only map() is a stream method - forEach() exists for both, streams and Collections/Iterables.