Question

I need a Map container that contains either an

Item Object

or

List<Item> Object

as its value, and I can get it out without casting, is it possible?

Answer 1

Short answer: No.

Union types don’t exist in Java. The closest thing you could do in order to get compile-time type checking would be to create list of some custom wrapper class which contained either an A or a List<A>, something like the following:

public class UnionListWrapper<A> {
    private final A item;
    private final List<A> list;

    public UnionListWrapper(A value) {
        item = value;
        list = null;
    }

    public UnionListWrapper(List<A> value) {
        item = null;
        list = value;
    }

    public Object getValue() {
        if (item != null) return item;
        else return list;
    }
}

At least you wouldn’t be able to create instances of this class that weren’t either an A or a List<A>, but getting the value back out would still have to just be Object, with associated casting. This could get quite clumsy, and on the whole it’s probably not worth it.

In practice I’d probably just have a List<Object> with some comments around being very careful about what data types are accepted. The problem is that even run-time checks (sub-optimal) aren’t going to be possible, since Java’s generic erasure means that you can’t do an instanceof A check at runtime (doubly so on the generic parameter of the List).