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

For your first question take a look at Why does ArrayList have "implements List"?

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To answer your second question

java.util.ArrayList a = Arrays.asList(stra);

as you mentioned Arrays.asList returns its own implementation of AbstractList and unfortunately creators of this code also named this class ArrayList. Now because we cant cast horizontally but only vertically returned array list can't be cast to java.utli.ArrayList but only to java.util.AbstractList or its super types like java.util.List that is why your first code example works.