What does List<?> mean in java generics?

java generics bounded-wildcard
corydoras picture corydoras · Dec 4, 2009 · Viewed 43.9k times · Source

What does List<?> mean, does it mean simply a list of objects of unspecified type?

Googling for the string <?> returns nothing useful (:

Answer

Pops picture Pops · Dec 4, 2009

As Tom said, the ?, or unbounded wildcard, means that the type of the object is not specified. It could be unknown, could be meant for multiple possible values or might be just plain irrelevant. Your example, List<?>, is pronounced "List of unknown." It's convenient because it's flexible, but there are also some pitfalls because you can't shove random objects in and pull them out of groups of unknown with total impunity.

Resources:

  • Wildcards are discussed here in the Java tutorial.
  • There's a good -- if verbose -- tutorial on generics in general by Angelika Langer available here.
  • And there's another good overview here (PDF) by Gilad Bracha; check out pages 5-7.
  • Finally, if you can get your hands on Effective Java by Josh Bloch, it's got a great section on generics and the cases in which you can, can't, should and shouldn't use wildcards (chapter 5, pages 109-146 in the second edition).

Incidentally, your Google search failed because Google doesn't truck with special characters:

With some exceptions, punctuation is ignored (that is, you can't search for @#$%^&*()=+[]\ and other special characters).

-Google help page

(EDIT: I must have been really tired when I wrote this last night. Cleaned up formatting/added a little info.)

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