In java generics what is the difference between the wildcard symbols: and ?

Question

I'm trying to learn generics and I had some questions about wildcards and their use.

What is the difference between the wildcards for upper and lower bounds and specifically what is the difference between: <? extends Object> and <? extends T> in java?

This seems a bit general. Did you study generics thoroughly first? You'd know it if you had - canonical tut - http://docs.oracle.com/javase/tutorial/java/generics/ — Caffeinated, Jun 05 '13 at 18:57
correct me if im' wrong - T is usually used in examples , in real life it should be a meaningful name... — Caffeinated, Jun 05 '13 at 18:58

score 1 · Accepted Answer · edited May 23 '17 at 12:28

Check here for your question on "upper and lower bounds" - SO link.

For an overview on generics check here

As for the difference between <? extends Object> and <? extends T>, the former is a type that is bound by the type java.lang.Object whereas the latter is bound by the type variable T - meaning you can satisfy by any type that extends java.lang.Object(which is all Java types) in the former and in the latter, you can satisfy by any type that extends the type T. Remember that T can be anything - even java.lang.Object.

score 0 · Answer 2 · edited Jun 06 '13 at 00:18

0

<? extends Object> is ALMOST useless. All that means is you're working with something that is not a primitive type. <? extends T> means you're working with an object that is of type T or a subclass of T and has whatever methods and fields T has.

<? extends T> is useful to declare what an Object is expected to contain. It limits the scope of the container or the method which adds funcionality to the method/container.

For Example:

public class foobination<T extends foo>
{
   private <? extends T> foo;

   public foobination(T foo)
   {
      this.foo = foo;
   }

   public getFoo() { return foo; }

   public void myFoobinator(<? extends T> foob)
   {
     foo.bash("foobinator was here");
     foo.setBang(foob.getBang());
   }
}

the other common use case would be: List<? extends T> Where you're storing a collection of something and you don't know exactly what it is but you do know they will all be a subclass of whatever T is. That way you at least have some idea of what to expect to get back out of the collection.

edited Jun 06 '13 at 00:18

Paul Bellora

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answered Jun 05 '13 at 18:59

Raystorm

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Generics don't work with primitives in Java – NullUserException Jun 05 '13 at 19:02
I stand corrected. I guess that means ` extends Object>` is useless after all, unless there is some other subtlety I'm missing. – Raystorm Jun 05 '13 at 19:05
@NullUserException Which is why the second sentence is correct, no? – Edward Thomson Jun 05 '13 at 19:05
1

@EdwardThomson The main problem I have with this answer is that it does little to explain what ` extends T>` does and why/where/when it should be used. – NullUserException Jun 05 '13 at 19:12
@NullUserException I agree that's the case, but I think your statement about generics falls in the same bucket: it's technically correct, which is admittedly the best kind of correct, but it's not particularly helpful to answering or elucidating. – Edward Thomson Jun 05 '13 at 19:51
1

@EdwardThomson Which is why it was just a comment – NullUserException Jun 05 '13 at 19:52

In java generics what is the difference between the wildcard symbols: and ?

2 Answers2