What's the difference between wildcard and 'T'? [duplicate]

Answer 1

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

means "do something with a list of any subclass of Animal".

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

means "do something with a list of some particular subclass (A.K.A. T) of Animal".

If I call list.get() on the first method, all I know about the returned type is that it extends Animal. On the second method, if I had another instance of List<T>, I know that whatever type T is, both lists would accept the same type.

To take it even further, if I say

Animal animal = list.get(0);

I can no longer say

list.add(animal);

even on the very same list, because we don't have a reference to the generic subtype. If however, I declare a List<T> where <T extends Animal>, I can now say

T animal = list.get(0);
list.add(animal);

because we know that list expects elements of type T.

Answer 2

Writing <T extends Animal> binds the type name T, so that it can be referred to later in the definition (including in the parameter list).

If you wrote <? extends Animal>, then you did not name the type. Therefore, you cannot refer to it later. You can't refer to it as ? later because that could be ambiguous (what if you had two type parameters?).

Java forbids you from writing public <?> ... because such a declaration is useless (the type parameter is not named so it cannot be used).