What is a "?" type in Java, eg. Future?

Question

I just stumbled upon Future<?>. What is the '?'

http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/AbstractExecutorService.html#submit(java.lang.Runnable)

http://docs.oracle.com/javase/tutorial/java/generics/capture.html — Brian Roach, May 27 '13 at 14:19
@LuiggiMendoza Not exactly "instance"---"any reference type" is more appropriate. — Marko Topolnik, May 27 '13 at 14:22
I don't think it's fixed :) You still imply a "reference" to name the placeholder for a *type*. There are no instances, nor references to them, involved. — Marko Topolnik, May 27 '13 at 14:24

score 5 · Answer 1 · edited May 23 '17 at 10:26

5

It is a wildcard expression used for generics.

See e.g.

http://docs.oracle.com/javase/tutorial/java/generics/capture.html

and

Difference between generic type and wildcard type

edited May 23 '17 at 10:26

Community

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answered May 27 '13 at 14:18

davek

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score 1 · Accepted Answer · answered May 27 '13 at 14:35

Future<?> is an unbounded wildcard reference and is short for Future<? extends Object>. Meaning the generic can be of any type.

You could also have Future<? extends T>, where it has to extend a specific type T (upper bound), or Future<? super T> where is must be an ancestor of a specific type T (lower bound).

http://docs.oracle.com/javase/tutorial/java/generics/wildcards.html

What is a "?" type in Java, eg. Future?

2 Answers2