Why both ArrayList and AbstractList are implements List interface?

[muthukrishnan.muthu]

Hi All,

I don't know the reason why the ArrayList & AbstractList classes are implements List interface as the AbstractList classes itseft it implements List interface.


AbstractList<E> extends AbstractCollection<E> implements List<E> {
ArrayList<E> extends AbstractList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable{

[jim829]

Well, unless there is something subtle that I am unaware of, it looks like a redundant declaration to me. You could still assign an ArrayList object to a List reference even if the ArrayList did not explicitly declare that it implements List. Perhaps others may have a different perspective.

An afterthought could be that it is simply a way to emphasize that ArrayList implements List (which is very useful to know). So it is somewhat self documenting as opposed to syntactically necessary.

Jim

[wsaryada]

As Jim said the reason can be for documentation especially for the generated Javadoc so that the ArrayList will be included as a class that implements the java.util.List interface. And the AbstractList was there to implements some general features shared by the java.util.List implementation to reduce the code duplications.