Why can't the Integer.class object be added to a Collection> object?

Question

I thought to have a quite good understanding of generics but just can't figure out why I get a compiler error here:

    Collection<Class<Number>> ncs = new HashSet<Class<Number>>();               
    ncs.add(Number.class);      //ok!
    ncs.add(Integer.class);     //compiler error here

Would be nice if anyone could explain this. :)

Edit: I understand why it's not possible to add a Integer.class object to a Collection> as it is pointed out in this question. But I don't understand why my example is the same. Of course this doesn't work:

Collection<? extends Number> ns = new HashSet<Number>();
ns.add(new Integer(1)); //compiler error here

But this does:

Collection<Number> ns = new HashSet<Number>();
ns.add(new Integer(1)); //ok!

As far as I understand my initial code uses the same concepts.

The type of `Integer.class` is `Class`, which is not compatible with `Class`. — Sotirios Delimanolis, Jun 01 '16 at 19:34
I just edited the question as I think it's another one than the one marked as duplicate. — Morrandir, Jun 01 '16 at 19:45
No, it's the same. A `Class` is not a `Class` just as a `List` is not a `List`. — Sotirios Delimanolis, Jun 01 '16 at 19:51
This is unrelated to the `Collection` you're trying to add to. You can remove that entirely and the problem would be the same. — Sotirios Delimanolis, Jun 01 '16 at 19:52

score 3 · Accepted Answer · edited May 23 '17 at 10:28

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Given classes X, Y, and Z, where X is a subclass of Y, X<Z> is a subclass of Y<Z> (e.g. what you've done with Collection and HashSet), but Z<X> is not a subclass of Z<Y>, so a Class<Integer> instance is not a Class<Number> and you can't put the former in a collection of the latter. For the latter you need wildcards, as romeara has shown.

See this question.

edited May 23 '17 at 10:28

Community

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answered Jun 01 '16 at 19:33

Alex Hall

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That is correct but not the answer to his problem. – sim Jun 01 '16 at 19:36
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@sim It may not be the solution, but it definitely answers the question of *why I get a compiler error here*. – shmosel Jun 01 '16 at 19:41
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@sim Why not? It perfectly answers the question ... – Seelenvirtuose Jun 01 '16 at 19:41
The problem is a missunderstanding, Integer is a subclass of Number. But Integer.class is not a subclass of Number.class, Thus an Integer.class "object" cannot be added to a generic list of type Number.class. – sim Jun 01 '16 at 19:44
@sim how is that not exactly what I've written? – Alex Hall Jun 01 '16 at 19:53

romeara · Answer 2 · 2016-06-01T19:38:47.810

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I believe you need to declare the collection as

Collection<Class<? extends Number>> ncs = new HashSet<Class<? extends Number>>();

because of what Alex Hall explained in his answer:

Given classes X, Y, and Z, where X is a subclass of Y, X<Z> is a subclass of Y<Z>, but Z<X> is not a subclass of Z<Y>

edited Jun 01 '16 at 19:38

answered Jun 01 '16 at 19:34

romeara

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This has nothing to do with erasure. – shmosel Jun 01 '16 at 19:36
@shmosel but the suggestion is still correct, right? – Alex Hall Jun 01 '16 at 19:36
@AlexHall yes it is. – shmosel Jun 01 '16 at 19:37
@AlexHall I did not. – shmosel Jun 01 '16 at 19:38
@AlexHall Although the suggestion is correct, the explaining sentence is so wrong that it leads to the wrong direction and makes it a bad answer. Hence the downvote. I also did that ... and right now I removed my downvote because of the edit. – Seelenvirtuose Jun 01 '16 at 19:38
Now I'm stuck in a recursive loop. – shmosel Jun 01 '16 at 19:39
I have fixed the explanation/terminology to have the correct explanation for future finders of the question. Thank you for pointing out the error! – romeara Jun 01 '16 at 19:39

Why can't the Integer.class object be added to a Collection> object?

2 Answers2