0.8 Xna0114-runtime polymorphism through class inheritance  (Page 2/7)

In some cases, multiple methods have the same name, but different formal argument lists. This is polymorphism implemented using overloaded methods. I explained this form of polymorphism in an earlier module.

In other cases, multiple methods have the same name, same return type, and same formal argument list. This is method overriding , which is the main topic of this module.

The essence of runtime polymorphic behavior

Methods are called on references to objects. Typically, those references are stored in reference variables, or in the elements of a collection such as anarray, stack, or queue.

The decision process

With runtime polymorphism based on method overriding , the decision regarding which version of a method will be executed is based on the actual type of the object whose reference is stored in the reference variable, and not on the type of the reference variable on which the method is called.

Stated differently, the type of the reference determines which methods can be called . The type of the object determines which method (from that set of allowable methods) will be called .

Late binding

The decision regarding which version of the method to call cannot be made at compile time. That decision must be deferred and made at runtime. This issometimes referred to as late binding .

Operational description of runtime polymorphism

Here is an operational description of runtime polymorphism as implemented in C# through inheritance and method overriding:

• Assume that a class named SuperClass defines a method named method .
• Assume that a class named SubClass extends SuperClass and overrides the method named method .
• Assume that a reference to an object of the class named SubClass is assigned to a reference variable named ref of type SuperClass .
• Assume that the method named method is then called on the reference variable using the following syntax: ref.method() .
• Result: The version of the method named method that will actually be executed is the overridden version in the class named SubClass . The version that is defined in the class named SuperClass will not be not executed.

This is runtime polymorphism.

Runtime polymorphism is very powerful

As you gain more experience with C#, you will learn that much of the power of OOP using C# is centered on runtime polymorphism using class inheritance,interfaces, and method overriding. (The use of interfaces for polymorphism will be discussed in a subsequent module.)

An important attribute of runtime polymorphism

This is worth repeating:

Why is it called runtime polymorphism?

The reason that this type of polymorphism is often referred to as runtime polymorphism is because the decision regarding which version of the method to execute cannot be made until runtime. The decision cannot be made at compiletime (as is the case with overloaded methods).