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

just wanna to complement answers to question 2

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

compiler just knows the return type of Arrays.asList is List, but does not know its exact implementation which may not be java.util.ArrayList. So you got this compile time error.

Type mismatch: cannot convert from List to ArrayList

you can force an upper cast explicitly like this,

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

The code will compile successfully, but a runtime exception will happen,

java.lang.ClassCastException: java.util.Arrays$ArrayList cannot be cast to java.util.ArrayList

this is because java.util.Arrays$ArrayList (the type of implemenation which Arrays.asList returns) is not a subtype of java.util.ArrayList.