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

Question

The implementation of java.util.ArrayList implements List as well as extends AbstractList. But in java docs you can see that AbstractL

Answer 1

1) ArrayList implements List is redundant but still legal. Only JCF (Java Collection Framework) designers could answer why. Since the lead JCF designer J.Bloch does not say why it's like this in "Effective Java" it seems we will never know why.

2) Arrays.asList returns

public class Arrays {
   ...

    private static class ArrayList extends AbstractList
        implements RandomAccess, java.io.Serializable
    {
...

it's not java.util.ArrayList and it cannot be cast to it