Java theory question

Question

Hi,
Why is it possible that we can declare a variable to be of an interface type and yet at the same time assign to it an object?
For example, I have an interface class (Interface), 1 class (FirstClass) that implements the interface and another 1 class (SecondClass) which not only implements the interface but also declares an instance variable to incorporate an interface object. (Concept of re-use by composition).
Subsequently, I have another class that declares a variable to be the interface type (Interface) and yet assign to it a (SecondClass) object. The code goes something like this:
Interface var = new SecondClass(new FirstClass());
How is it possible that I can access all the methods found in FirstClass?
Thanks in advance.

Answer 1

You can't use the non-interface methods in FirstClass. The only thing that's important when you writeList list = new LinkedList ();is that you're interested in the methods (and fields) that are defined in the interface. You can't use removeFirst (which is in LinkedList), but you're not interested in it.