为什么将不可修改的集合保护到“集合”包中？

Question

I cannot fathom why Java wishes to hide immutable unmodifiable collections from being passed around.

It makes life harder on devs because you can't guarantee that a class is returning anything immutable unmodifiable, forcing you to smatter calls to Collections.unmodifiableWhatever throughout your code. This is both wasteful and annoying.

Is there a reason I'm missing behind why you would make these methods protected, or another library I'm missing that contains public versions of immutable and/or unmodifiable collections?

For the purposes of learning more about java, let's say Scala does not count as an answer to "a library that contains public versions of immutable collections" :)

Answer 1

I think it has to do with the design goals of the framework:

The main design goal was to produce an API that was small in size and, more importantly, in "conceptual weight."

( Source )

You should check out Guava's immutable collection types , if you are willing to learn more conceptual weight :)

Answer 2

The Collections interface permits one to wrap an exiting Collection so that calls to mutator methods result in failure.

unmodifiableCollection(Collection c) : "Returns an unmodifiable view of the specified collection."

It makes life harder on devs because you can't guarantee that a class is returning anything immutable, ... This is both wasteful and annoying.

The JDK source of the Collections.unmodifiableList() method is:

public static <T> List<T> unmodifiableList(List<? extends T> list) {
    return (list instanceof RandomAccess ?
            new UnmodifiableRandomAccessList<>(list) :
            new UnmodifiableList<>(list));
}

I see why this can be considered wasteful (these generics also make it ugly). Why doesn't that method check whether the passed instance is already an UnmodifiableRandomAccessList or UnmodifiableList ?