What is the purpose of AbstractCollection

Question

AbstractCollection implements Collection. So why is AbstractCollection there and why do we use Collection instead of directly using AbstractCollection?

Answer 1

Maybe it is a little late to answer but just in case I want to answer this question.

1. AbstractCollection is used since all the child collections share some functionalities in common as Krzysztof said and, since this functionality is common (exactly the same lines of code) we can use inheritance to rehuse that code and make the design cleaner, easier to understand and more maintainable.

2. We use the interface since an interface defines the "contract" that every class implementing the interface must provide, i.e the methods that the subclasses must have. This question is similar to, why can't we use ArrayList instead of List, you can do that, but 90% of the times you will be interested on adding/setting, deleting and knowing the size of an ArrayList and the List interface assures you that anything that is written after a new somewhere will implement this methods (and others) so you can call them and polymorphism will do the work. Also, using the interface as a reference allows you to change the type of the concrete implementation without changing anything (only the new calls) and also allows you to change the specific type at runtime, using the specific class reference you are constraining yourself to that reference and only that.

Answer 2

Adding to what @Angelixus says in https://stackoverflow.com/a/58513874/139985:

One reason to NOT do this:

AbstractCollection col = (AbstractCollection) someCollection;

is that subtypes of Collection are NOT guaranteed to extend the AbstractCollection base class.

It is certainly the case that most (if not all) of the standard collection types do extend this base class. However, this may not apply to all 3rd-party collection classes. If you encounter a collection class that is implemented without extending AbstractCollection, the above will give either a compilation error or a runtime exception, depending on the declared type of someCollection.