Why can't Stream.flatMap accept a collection?

Question

Given the following as an example of data classes:

class Country {

    List regions = new ArrayList<>();

    List getRegi         


        

Answer 1

A technical reason, which is not ideal but could be why this wasn't done. You can't overload on a generic type in Java.

They need to support

 Stream.flatMap(Function<Object, Stream<X>> function)

which means they can't overload it with

 Stream.flatMap(Function<Object, Collection<X>> function)

as these two methods have the same signature after erasure.

They could add a method

 Stream.flatMapCollection(Function<Object, Collection<X>> function)

or

 Stream.flatMapIterable(Function<Object, Iterable<X>> function)

 Stream.flatMapI(Function<Object, Iterable<X>> function)

but it wouldn't be pretty.