2.7 Java1614: polymorphism and the object class

Revised: Thu Mar 31 13:07:54 CDT 2016

This page is included in the following Books:

• ITSE 2321 - Object-Oriented Programming (Java)
• Object-Oriented Programming (OOP) with Java

Table of contents

• Preface
  • Viewing tip
    • Listings
• Preview
• Discussion and sample code
• Summary
• What's next?
• Miscellaneous
• Complete program listing

Preface

This module is one of a series of modules designed to teach you about the essence of Object-Oriented Programming (OOP) using Java.

Viewing tip

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

Listings

• Listing 1 . Definition of the class named A.
• Listing 2 . Definition of the class named B.
• Listing 3 . Definition of the class named C.
• Listing 4 . Beginning of the class named Poly04.
• Listing 5 . A new object of the class named B.
• Listing 6 . A new object of the class named C.
• Listing 7 . Complete program listing.

Preview

What is polymorphism?

If you have studied the earlier modules in this collection, you should already know what polymorphism is, how it is implemented in Java, the threedistinct forms of polymorphism in Java, etc.

I discussed runtime polymorphism implemented through method overriding and class inheritance in a previous module. However, before leaving that topic, I need to discuss an important special case.

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

I will briefly discuss the default versions of eleven methods defined in the Object class, and will explain that in many cases, those default versions are meant to be overridden.

Discussion and sample code

The Java Collections Framework

Java supports a framework, known as the Java Collections Framework, which you can read about here .

Without getting into a lot of detail, the framework provides several concrete implementations of interfaces with names like list , set , and map .

The classes that provide the implementations have names like LinkedList , TreeSet , ArrayList , Vector , and Hashtable . As you might recognize, the framework satisfies the requirements for what we might refer to as classical data structures.

Not the purpose ...

However, it is not the purpose of this module to discuss either the Java Collections Framework, or classical data structures. Rather, they are mentioned here simply because the framework provides a good example of the use of the Object class as a generic type for runtime polymorphic behavior.

(Also beyond the scope of this module is the fact that the framework provides an outstanding example of the implementation of polymorphic behavior through the use of the Java interface. The use of the Java interface is a topic for a future module)