ArrayList vs ArrayList

Question

Suppose I have:

class A{
}
class B extends A{
}
class C extends A{
}

Why and when we want to use ArrayList<? extends A>, Why not simply write ArrayList<A> ?

So why don't you create two lists and add A's and B's to it? — NomadMaker, Feb 27 '20 at 20:28

score 1 · Answer 1 · answered Feb 27 '20 at 20:46

Consider that you are not the one creating the List, you are getting it passed to you from somewhere. Say you have a method that takes an argument List<? extends A>. Someone could call your method and pass to you a List<B> that he created. However, if you declared your argument to be just List<A>, he would not be allowed to pass his List<B> to you.

Here's why: In the first case, where your argument is List<? extends A>, you are not allowed to add to this list because you don't know how the caller created it. If you added, say, an instance of C and returned, he would now have a C object in his List<B>. If it were a real List<A>, you'd be able to add to it safely.

Here's why: Because `List` and `List` are unrelated. [Is List a subclass of List? Why are Java generics not implicitly polymorphic?](https://stackoverflow.com/q/2745265/5221149) — Andreas, Feb 27 '20 at 20:50

ArrayList vs ArrayList

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