How to manage a collection for a type which extends another?

Question

I have the following situation:

public class A {...};
public class B extends A {...};

And I defined a function inside a class C with the following header:

private void handleABC(final Collection<A>) {...}

but I get the following message when I try to call it passing type B:

The method handleABC(Collection) in the type C is not applicable for the arguments (Collection).

Should't this work for both A and B since I defined the method Collection<A> and B extends from A? What am I doing wrong?

You can use `Collection extends A>` in your `handleABC` declaration. — Thoomas, Jun 27 '18 at 09:02
@Berger `C c = new C(); c.handleABC(Arrays.asList(new B()));` works fine for me because `B` is a type of `A` — Shubhendu Pramanik, Jun 27 '18 at 09:20

score 3 · Accepted Answer · answered Jun 27 '18 at 09:05

3

B extends A, yes, but Collection<B> extends Collection<A> is not the case.

As someone mentioned in a comment, try Collection<? extends A>.

answered Jun 27 '18 at 09:05

Robert

score 2 · Answer 2 · answered Jun 27 '18 at 09:05

2

change the handleABC() signature to

private void handleABC (final Collection<? extends A> )

so that the methods accepts Collection of subclasses of A.

answered Jun 27 '18 at 09:05

Mihir

Cuong Bui · Answer 3 · 2018-06-27T09:57:02.997

0

You must pass the class specified in diamond operator, not concrete classes or you can use "? extends A". You can read this link : https://docs.oracle.com/javase/tutorial/java/generics/inheritance.html

edited Jun 27 '18 at 09:57

answered Jun 27 '18 at 09:09

Cuong Bui

score 0 · Answer 4 · answered Jun 27 '18 at 09:10

0

<T extends SomeClass>

when the actual parameter can be SomeClass or any subtype of it.

In your case

private void handleABC (final Collection<? extends A> )

answered Jun 27 '18 at 09:10

Ashu

4 Answers4