CODEWITHSUNDEEP
javacollectionsextends
Rexam
Rexam

Reputation: 913

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


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?

Upvotes: 1

Views: 104

Answers (4)

Cuong Bui
Cuong Bui

Reputation: 36

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

Upvotes: 0

Ashu
Ashu

Reputation: 2266

Please review this answer: https://stackoverflow.com/a/897973/3465242

<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> )

Upvotes: 0

Mihir
Mihir

Reputation: 581

change the handleABC() signature to

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

so that the methods accepts Collection of subclasses of A.

Upvotes: 2

Robert
Robert

Reputation: 307

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

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

Upvotes: 3

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