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

I believe, there is a reason. This is just my thought and I didn't find it anywhere in JLS.

If I am a developer who is writing an API which is to be widely used, why will I do this?

There is absolutely no reason of doing this, but consider this scenario, where I have written the List interface and provided the ArrayList implementation for the the List interface.

I have not yet written any abstract class AbstractList till now.

One day a requirement comes, where I am asked to write few more implementations of the List interface where most of them are having similar or same concrete methods for the abstract methods in List interface.

I will go ahead and write an AbstractList with necessary implementation for all those methods. But now I will not like that half of my classes to implement the List interface and half of them extending AbstractList.

Also, I cannot just go and remove the 'implements List` from the classes I wrote earlier, might be because this is not the right time or I do not want other's code to break with my new release.

Note This is solely my opinion.