Question

Using Java 6 SE, I have a class that contains a Vector. There is a class method that simply calls the contained Vector’s addAll() method with the same parameters for each. The Vectors can be of either type “Superclass” or type “Subclass” and I want the addAll() methods to support both. This whole setup might look like this:

public class VectorAContainer(){
    this.vectorA = new Vector<Superclass>();
}

public void addAll(Vector<Superclass> vector){
    this.vectorA.addAll(vector);
}

Then I want to call a line of code like this:

VectorAContainer container = new VectorAContainer();
container.addAll(vectorB); //where vectorB is a Vector<Subclass>

The problem is, Eclipse gives the following error:

“The method addAll(Vector<Superclass>) in the type
VectorAContainer is not applicable for the arguments
(Vector<Subclass>).”

Strangely, I have found that addAll() works if it’s NOT inside of a class:

//No container with an addAll(), calling directly on vectorA
this.vectorA.addAll(vectorB); //No errors!

I don’t see a difference between the two above cases. I would like to get the first way to work because I have a much more elaborate class that contains a Vector like the one above. I would greatly appreciate any help you all could provide!

Answer 1

You need to accept a covariant generic parameter:

public void addAll(Collection<? extends Superclass> c)

You can see this in the declaration of addAll:

boolean addAll(Collection<? extends E> c)