Java, A Beginner's Guide, 5th Edition, 5th Edition

Using Interface References

You might be somewhat surprised to learn that you can declare a reference variable of an interface type. In other words, you can create an interface reference variable. Such a variable can refer to any object that implements its interface. When you call a method on an object through an interface reference, it is the version of the method implemented by the object that is executed. This process is similar to using a superclass reference to access a subclass object, as described in Chapter 7.

The following example illustrates this process. It uses the same interface reference variable to call methods on objects of both ByTwos and ByThrees.

In main( ), ob is declared to be a reference to a Series interface. This means ...