2.6 Java1612: runtime polymorphism through inheritance

Revised: Thu Mar 31 12:36:11 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 . Beginning of the driver class named Poly03.
• Listing 4 . Polymorphic behavior.
• Listing 5 . Source of a compiler error.
• Listing 6 . A new object of type A.
• Listing 7 . Complete program listing.

Preview

What is polymorphism?

The meaning of the word polymorphism is something like one name, many forms .

How does Java implement polymorphism?

Polymorphism manifests itself in Java in the form of multiple methods having the same name.

In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods, which were discussed in a previous module) .

In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods) .

Three distinct forms of polymorphism

From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

• Method overloading
• Method overriding through class inheritance
• Method overriding through the Java interface

I covered method overloading as one form of polymorphism (compile-time polymorphism) in a previous module. I also explained automatic type conversion and the use of the cast operator for type conversion in a previous module.

In this module ...

I will begin the discussion of runtime polymorphism through method overriding and class inheritance in this module. I will cover interfaces in afuture module.

The essence of runtime polymorphic behavior

With runtime polymorphism based on method overriding,

• the decision as to 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.

Late binding

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

Discussion and sample code

Operational description of runtime polymorphism

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