Why does arraylist class implement List as well as extend AbstractList?

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Then wouldn’t it be redundant to implement List as well as extend AbstractList?

Yes, it is 100% redundant. However, Java implementors added interfaces very consistently in all public implementation of the collections library:

  • LinkedList<E> and ArrayList<E> extend AbstractList<E> which implements List<E>, and then implement List<E> themselves.
  • HashSet<E> and TreeSet<E> extend AbstractSet<E> which implements Set<E>, and then implement Set<E> themselves.
  • HashMap<K,V> and TreeMap<E> extend AbstractMap<K,V> which implements Map<K,V>, and then implement Map<K,V> themselves.

My understanding is that they did so for documentation purposes: the authors wanted to show that ArrayList<E> is primarily a List<E>; the fact that ArrayList<E> extends AbstractList<E> is a less significant detail of its implementation. Same goes for the other public collection types.

Note that Arrays.ArrayList<E> class is not publicly visible, so its authors did not care to include List<T> explicitly.

As far as the failed conversion goes, this should come as no surprise, because the inner class Arrays.ArrayList<E> and the public class ArrayList<E> are unrelated to each other.

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