0.9 Xna0116-runtime polymorphism and the object class

Revised: Sat May 07 11:13:14 CDT 2016

This page is part of a Book titled XNA Game Studio .

Table of Contents

• Table of Contents
• Preface
  • Viewing tip
    • Figures
    • Listings
• General background information
  • The generic type Object
  • References of type Object
  • Methods in the Object class
  • The difference between string and String
  • Every class inherits these methods
  • To be overridden...
  • Calling methods of the Object class
  • And the behavior will be...
• Preview
• Discussion and sample code
  • The class named A
    • Does not override the ToString method
  • The class named B
    • Overrides the ToString method
    • Purpose of the ToString method
    • Can be overridden
    • Behavior of the default and overridden versions
    • Will be useful later
  • The class named C
    • Behavior of overridden version
  • The driver class
    • A new object of the class A
      • Call ToString method on the reference
      • Display the returned String
      • Default ToString behavior
      • Class A does not override ToString
    • A new object of the class B
      • Call ToString and display the result
      • Do you recognize this?
      • Overridden version of ToString was executed
      • Once again, what is the rule?
    • An object of the class C
      • What will the output look like?
      • Overridden version of ToString was called
  • No downcasting was required
  • A generic array object
  • Collections
• Run the program
• Run my program
• Summary
• Miscellaneous
• Complete program listing

Preface

This module is one in a collection of modules designed primarily for teaching GAME 1343 Game and Simulation Programming I at Austin Community College in Austin, TX. These modules are intended tosupplement and not to replace the textbook.

An earlier module titled Getting Started provided information on how to get started programming with Microsoft's XNA Game Studio.

Object-oriented programs exhibit three main characteristics:

• Encapsulation
• Inheritance
• Polymorphism

I have explained encapsulation, inheritance, compile-time polymorphism, and runtime polymorphism using method overriding and class inheritance in earliermodules. This module will explain the importance of the Object class in polymorphic behavior. I will defer an explanation of polymorphism usinginterface inheritance until a future module.

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figuresand Listings while you are reading about them.

Figures

• Figure 1 . Text output for ToString method and class A.
• Figure 2 . Text output for overridden ToString method and class B.
• Figure 3 . Text output for overridden ToString method and class C.

Listings

• Listing 1 . Definition of class A.
• Listing 2 . Definition of class B.
• Listing 3 . Definition of class C.
• Listing 4 . Beginning of the driver class.
• Listing 5 . A new object of the class B.
• Listing 6 . An object of the class C.
• Listing 7 . Project Polymorph04.

General background information

The generic type Object

In this module, I will explain the use of the Object class as a completely generic type for storing references to objects of subclasstypes, and will explain how that results in a very useful form of runtime polymorphism.