Home/ Questions/Q 8858963
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-14T14:56:43+00:00

A book I am reading on Java tells me that the following two pieces

  • 0

A book I am reading on Java tells me that the following two pieces of code are equivalent:

public <T extends Animal> void takeThing(ArrayList<T> list)

public void takeThing(ArrayList<? extends Animal> list);

On the opposite page, I am informed that the latter piece of code uses the ‘?’ as a wildcard, meaning that nothing can be added to the list.

Does this mean that if I ever have a list (or other collection types?) that I can’t make them simultaneously accept polymorphic arguments AND be re-sizable? Or have I simply misunderstood something?

All help/comments appreciated, even if they go slightly off topic. Thanks.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    Does this mean that if I ever have a list (or other collection types?) that I can’t make them simultaneously accept polymorphic arguments AND be re-sizable?

    No.

    The two pieces of code are not completely equivalent. In the first line, the method takeThing has a type parameter T. In the second line, you’re using a wildcard.

    When you would use the first version, you would specify what concrete type would be used for T. Because the concrete type is then known, there’s no problem to add to the list.

    In the second version, you’re just saying “list is an ArrayList that contains objects of some unknown type that extends Animal“. What exactly that type is, isn’t known. You can’t add objects to such a list because the compiler doesn’t have enough information (it doesn’t know what the actual type is) to check if what you’re adding to the list should be allowed.

    • 0