Enum as instance variables

If you have an enum such as

enum Coffee {
    BIG,
    SMALL
}

and a class that has an instance variable like this of the enum:

public class MyClass {
    private Coffee coffee;

    // Constructor etc.
}

Why is it possible in the constructor to say e.g. coffee.BIG? I don't understand that you can use the reference? Is enum as instance variables initialized to something other than null? It is the self test question #4 in the SCJP book in the first chapter. I tried to shorten the code and question.