Question

Possible Duplicate:
Why does ArrayList have “implements List”?

I am new to java I was trying to see the hierarchy of collection interface. I found that the signature of AbstractList.java is like

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E>

It implements List interface. But if you look at signature of child class ArrayList.java it looks like

public class ArrayList<E> extends AbstractList<E>   implements List<E>, RandomAccess, Cloneable, java.io.Serializable

If you look parent class is already implemented List interface then why child class is again implementing same interface (List).

Is there specific reason or requirement behind this

Answer 1

Is there specific reason or requirement behind this

I don’t think so. Removing the second List<E> would change absolutely nothing as far as the compiler is concerned.

My two hypotheses are:

1. It’s there for historic reasons. However, I can’t find any evidence to support this.
2. It’s included for documentation purposes, to make it immediately clear than an ArrayList<E> is-a List<E> (this is probably the single most important thing to know about ArrayList).