How to properly implement java.util.Collection class ?

Question

I want to write my own Linked-list< T> and implement java.util.Collection< T>.

My problem is the warning: "The type parameter T is hiding the type T". that occures when I override the method public <T> T[] toArray(T[] arg0){}

Here's my code:

public class MyLinkedList<T>  implements Serializable,Iterable<T>, Collection<T>
{
    //some constructor here.  

    public <T> T[] toArray(T[] arg0)  // I get that error here under the <T> declaration
    {
        return null;
    }
    ...
    // all other methods 
    ...
}

(I know I can extend AbstractCollection class instead but that's not what I want to do).

Anybody have any idea how to solve this?
Should I change the parameter T in Collection< T> to be some other letter like so: Collection< E> ?

Answer 1

You get this error because method <T> T[] toArray(T[] arg0) takes a generic parameter of its own, which is independent of the generic parameter T of your class.

If you need both types T (of the class) and T (of the method) to be available inside toArray implementation, you need to rename one of these types. For example, Java reference implementations use E (for "element") as the generic type argument of collection classes:

public class MyLinkedList<E>  implements Serializable, Iterable<E>, Collection<E>
{
    //some constructor here.  

    public <T> T[] toArray(T[] arg0)
    {
        return null;
    }
    ...
    // all other methods 
    ...
}

Now the names of the two generic parameters are different, which fixes the problem.