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

Superclass References and Subclass Objects

As you know, Java is a strongly typed language. Aside from the standard conversions and automatic promotions that apply to its primitive types, type compatibility is strictly enforced. Therefore, a reference variable for one class type cannot normally refer to an object of another class type. For example, consider the following program:

Here, even though class X and class Y are physically the same, it is not possible to assign an X reference to a Y object because they have different types. In general, an object reference variable can refer only to objects of its type.

There is, however, an important exception ...