CODEWITHSUNDEEP
java
Devy
Devy

Reputation: 703

ArrayList<? extends A> vs ArrayList<A>

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> ?

Upvotes: 0

Views: 44

Answers (1)

Zag
Zag

Reputation: 638

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.

Upvotes: 1

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