If list is an interface, then how come it has defined methods with full body?

Hi Soumitra,

Good question!

You're correct that List is an interface. However, ArrayList does not inherit List, it implements List. Casting involves inheritance and because ArrayList does not inherit List, there is no upcasting. Inheritance is the term that is used when a subclass extends an abstract class. We see the the inheritance chain at the top left hand corner of the Java docs for ArrayList (https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html). The ArrayList class inherits from java.util.AbstractList<E>.

What is happening when you write List<int> s=new ArrayList<int>() is implementation of an interface. You're correct that List has "abstract" (it's not really abstract because it doesn't use the abstract key word) method signatures. ArrayList implements the List interface and by doing so guarantees that it has written the body of these "abstract" methods (this is called implementing the interface).

So, to recap, classes can implement interfaces and extend/inherit abstract classes. Because List is an interface, ArrayList implements List and ArrayList writes the body of the "abstract" method signatures contained in List.

You're asking good questions! I hope my answer wasn't confusing, but let me know if it was or if you have more questions!

Keep at it!