Java Interfaces and their use in Collections

Question

I'm developing a Java library and I need to create a method which returns a Set<A>, where A is an interface and B implements A. So I tried instantiating the set like:

Set<A> setOfA = new HashSet<B>();

getting the following error:

Type mismatch: cannot convert from HashSet<B> to Set<A>

However, when not using a collection and returning B in a function which expects A as return type everything is ok, so the relation between the interface and its concrete class is ok and the problem is with collections, the HashSet in this case. How can I avoid this? Thanks in advance.

Should be `new HashSet();` or `new HashSet<>();`. You can still store B inside these HashSets. — Compass, Dec 05 '16 at 20:33
If you need to return a `Set`, why are you instantiating a `HashSet`? — shmosel, Dec 05 '16 at 20:36
@shmosel No, I'm not confused about inheritance "direction" and I know that, in my case, a `Set` is not a `Set`. @Compass You're right, it works in this way, I'll continue the discussion about this in the similiar answer. — RVKS, Dec 05 '16 at 20:51
As far as I can tell, you're asking why `HashSet` is not assignable to `Set`, even though `B` is assignable to `A`. If that's not your question, please clarify. — shmosel, Dec 05 '16 at 20:56
Yes I'm sorry, I was strongly differentiating _being the same_ with _being assignable to_, but I'm now understanding better thanks to the linked question's answers. — RVKS, Dec 05 '16 at 22:28

score 1 · Accepted Answer · answered Dec 05 '16 at 20:35

You can do it like this only:

Set<A> setOfA = new HashSet<>();

It's a short way of this:

Set<A> setOfA = new HashSet<A>();

So whan you need to use special methods from B, just do cast like this (if you are sure, that this object is really instance of B)

B b = (B) setofA.get(....);

You can read pretty good tutorial here. Good luck

Java Interfaces and their use in Collections

1 Answers1