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Why don't you just use a Set?

Why do you need to extend ArrayList in the first place?

If you need to keep track of two separate types (as opposed to simply keeping track of a single parent type), then you either need two...

It won't. It'll return an A reference, which you can cast to the appropriate type. If you really need to be able to return a B or C from an ArrayList, you're going to have to keep track of two...

Can't you just use an ArrayList of A?

ArrayList<A> list = new ArrayList<A>();