2.9 Java1618: polymorphism and interfaces, part 2

Revised: Thu Mar 31 15:07:17 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 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 interfaces named I1 and I2.
• Listing 2 . Definition of the class named A.
• Listing 3 . Definition of the class named B.
• Listing 4 . Definition of the class named C.
• Listing 5 . Beginning of the class named Poly06.
• Listing 6 . Try unsuccessfully to call the method named q.
• Listing 7 . Successfully call the method named q.
• Listing 8 . Instantiate a new object of the class B.
• Listing 9 . Try unsuccessfully to call the method named x.
• Listing 10 . Successfully call the method named x.
• Listing 11 . Call the toString method.
• Listing 12 . Try unsuccessfully to call the method named p.
• Listing 13 . Successfully call the method named p.
• Listing 14 . A walk in the park.
• Listing 15 . Complete program listing.

Preview

Method overloading

I covered method overloading as one form of polymorphism (compile-time polymorphism) in a previous module.

Method overriding and class inheritance

I discussed runtime polymorphism implemented through method overriding and class inheritance in more than one previous module.

Using the Java interface

In this and the previous module, I am explaining runtime polymorphism as implemented using method overriding and the Java interface.

A very important concept

In my opinion, this is one of the most important concepts in Java OOP, and the one that seems to give students the greatest amount of difficulty. Therefore, I am trying to take it slow and easy. As usual, I am illustrating the concept using sample programs.

A skeleton program

In the previous module, I presented a simple skeleton program that illustrated many of the important aspects of polymorphic behavior based on the Java interface.

Multiple inheritance and the cardinal rule

I explained how the implementation of interfaces in Java is similar to multiple inheritance. I explained the cardinal rule of interface implementation.

A new relationship

I explained that objects instantiated from classes that implement the same interface have a new relationship that goes beyond the relationship imposed by the standard class hierarchy.

One object, many types

I explained that due to the combination of the class hierarchy and the fact that a class can implement many different interfaces, a single object in Java can be treated as many different types. However, for any given type, there are restrictions on the methods that can be called on the object.