Suppose you're a store clerk, and a customer asks you, "what kinds of credit cards do you accept?"
The difference between List<Object> and List is basically the difference between answering this question "we accept all kinds," and answering it, "duuuuhhhhhhh?"
Ah, but what about List<Object> and List<?>? Or am I just trying to take this metaphor too far!?
ReplyDeleteList<?> is harder to explain since it's not the specific type of any instance, only the type of a _reference_; the type that can refer to List<Foo> for any Foo. I don't know how to work it into the analogy. :)
ReplyDeleteWhat about List<? extends Object>? Would that be like accepting Discover?
ReplyDeleteSurely List<? extends Object> is equivalent to List<?>. As I understand it, List<?> is just a shorthand that omits the extends clause, and so it defaults to Object as the root of the class hierarchy.
ReplyDeleteHow we work it into the analogy though is beyond me!
This is way outside of the analogy, but
ReplyDeleteList<? super Object>
is a mind-bender.
It is a zen thing: the store is open with employees, but doesn't accept anything.
Actually, having compiled an example, it may accept Object. Is Object a super of itself?
Yes, the true meaning of "super" in a wildcard expression is "assignable from". Likewise, "extends" here means "assignable to". These relationships are indeed reflexive.
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