javagenericsjvmbounded-wildcard

Why Wild Cards can't be used in generic class & method declaration?

Declaration like this :

  class A<X extends Number & List> {  }

is allowed.Whereas declaration like this is not allowed.

  class A<? extends Number & List> {  }

Is there any logical explanation about why Java restricts us to do that?

& what's the actual difference between 

      <T extends Number>  
     & <? extends Number>?
Solution

  • The whole point of a type parameter like T is so that you can use it as a type inside the class. What would a wildcard there even mean? If you can't use it anywhere, why have a type parameter at all?