Difference betwen returning the list interface or List implementation.

Craig, I think, is touching on the principle of Polymorphism. I'm not going to attempt to explain this in any depth (it's a fairly large, but interesting, topic), but what I will say is:

An interface is a contract. For example, the collection classes ArrayList, Vector, and LinkedList are all implementations that adhere to the List contract. That is, they agree to implement a specific set of methods, or in other words, behave in the same way.

This means if you have a method that uses an argument of an interface type then any object that meets the contract terms can be used in its place. For example, because an ArrayList implements the List interface, you can pass this into (or it return from, see example below) a method that takes a List argument.

List<String> getAllAuthors() {
    ArrayList<String> authors = new ArrayList<>();
    ...
    return authors;
}

One benefit of this is you can have an initial implementation of your method that uses an ArrayList, but change it later to use an alternative implementation, e.g. a LinkedList as both implementations behave the same way, without breaking the code (Craig's original point).

Hope this helps. iain

Ohhhh....the interface is the requirements, the contract, what must someone who claims to implement it must do. The implementation is the actual code that must have the methods defined in the interface.

That help?

so basically the Interface holds methods that must be overridden in the Class if it is implementing that Interface in order to adhere to the contract?