Java How to Program, Early Objects, 11th Edition

10.6 Allowed Assignments Between Superclass and Subclass Variables

Now that you’ve seen a complete application that processes diverse subclass objects polymorphically, we summarize what you can and cannot do with superclass and subclass objects and variables. Although a subclass object also is a superclass object, the two classes are nevertheless different. As discussed previously, subclass objects can be treated as objects of their superclass. But because the subclass can have additional subclass-only members, assigning a superclass reference to a subclass variable is not allowed without an explicit cast—such an assignment would leave the subclass members undefined for the superclass object.

We’ve discussed three proper ways to assign superclass ...