Generics wildcard

Question

I was wondering why the following bit of code does not work:

Collection <? super String> col = new ArrayList<String>();
col.add(new Object());// does not compile
col.add("yo!");// compiles indeed;

If the type is <? super String> it can contain anything which is super of String (String included) not?

` super String>` can only add String. `Object` can be not String.. Try to do: `Object myObj = new String();` and it should work. — Maroun, Jul 02 '13 at 07:59
No, anything that **extends** String. It means *? whose super is String*. — m0skit0, Jul 02 '13 at 08:01
@m0skit0 Ops.. Why If I then try to do ` super String> col = new ArrayList — Rollerball, Jul 02 '13 at 08:26
Relevant: http://stackoverflow.com/questions/12604477/use-of-extends-and-super-in-collection-generics/12605337#12605337 — Brian, Jul 02 '13 at 15:10

Marko Topolnik · Accepted Answer · 2013-07-02T08:13:33.410

15

Collection<? super String>, counter to your intuition, does not mean "a collection which contains objects of type String or its supertype". It means "col will be a collection holding some definite type, which itself is String or its supertype" (such as Object, Serializable, or CharSequence).

The best way to think about Collection<? super String> is that it is not a type, like you are used to in Java, but a pattern against which specific types are matched.

Now, the only thing you can safely add to any collection which matches the above pattern is a String or its subclass (if there were any). Quite the opposite of what you would have expected, isn't it? That's Generics for you.

edited Jul 02 '13 at 08:13

answered Jul 02 '13 at 08:02

Marko Topolnik

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Heh, nice. To complete your example, a `String` could be placed into a `Collection super CharSequence>`... – Lukas Eder Jul 02 '13 at 08:09
Thank you for the clear explanation Marko, it's not the first time your answer helps me – Plain_Dude_Sleeping_Alone Jan 18 '16 at 08:45

score 3 · Answer 2 · answered Jul 02 '13 at 08:02

With a Collection <? super String>, we don't know what kind of objects it contains exactly, but we know that it must be a collection of either String or some superclass of String, i.e. it is safe to put a String into it but not necessarily an Object. Conversely for methods that take things out of the collection (e.g. when iterating) we can't be sure we'll get strings back.

On the other hand with Collection<? extends Foo> we know it's a collection of something that is either Foo or some subclass of Foo, so we can safely take something from the collection and know it will be assignable to Foo, but we can't put anything in because we have no way of knowing what types would be safe.

Great explanation, I understand better now~ – Huibin Zhang Jun 08 '15 at 12:26 — Huibin Zhang, Jun 08 '15 at 12:26

Generics wildcard

2 Answers2