Polymorphism - Java Software Solutions: Foundations of Program Design - page 489

SELF-REVIEW QUESTIONS (see answers in Appendix N)
SR 10.1 What is polymorphism?
SR 10.2 Why is compile time binding considered more efficient than dynamic
binding?

10.2 Polymorphism via Inheritance

When we declare a reference variable using a particular class name, it can be used
to refer to any object of that class. In addition, it can also refer to any object of
any class that is related to its declared type by inheritance. For example, if the
class Mammal is the parent of the class Horse , then a Mammal reference
can be used to refer to any object of class Horse . This ability is shown
in the following code segment:

KEY CONCEPT
A reference variable can refer to any
object created from any class related
to it by inheritance.

Mammal pet;
Horse secretariat = new Horse();
pet = secretariat; // a valid assignment

The reverse operation, assigning the Mammal object to a Horse reference, can
also be done but it requires an explicit cast. Assigning a reference in this direc-
tion is generally less useful and more likely to cause problems, because although
a horse has all the functionality of a mammal (because a horse is-a mammal), the
reverse is not necessarily true.
This relationship works throughout a class hierarchy. If the Mammal class were
derived from a class called Animal , the following assignment would also be valid:

Animal creature = new Horse();

Carrying this to the limit, an Object reference can be used to refer to any
object, because ultimately all classes are descendants of the Object class. An
ArrayList , for example, uses polymorphism in that it is designed to hold
Object references. That's why an ArrayList that doesn't specify an element
type can be used to store any kind of object. In fact, a particular ArrayList
can be used to hold several different types of objects at one time, because, by
inheritance, they are all Object objects.
The reference variable creature can be polymorphic, because at
any point in time it can refer to an Animal object, a Mammal object,
or a Horse object. Suppose that all three of these classes have a
method called move that is implemented in different ways (because

KEY CONCEPT
The type of the object, not the type
of the reference, is used to determine
which version of a method to invoke.