INH - Polymorphism (Lesson)

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Polymorphism

Introduction

Poly means many. Polymorphism means many forms. With inheritance, we can have many methods that have the same name in different classes. Some of the methods will be overridden, having the same signature, while others will be overloaded, having a different parameter list. Polymorphism allows us to assign different meanings or implementations of something, such as a method, while sharing the idea throughout an entire inheritance hierarchy.

When a method is overwritten in at least one subclass, it is called polymorphic. It is possible for objects to be polymorphic as well.

Animal ed = new Animal (); ed = new Dog (); ed = new Kangaroo (); 

Assuming Dog and Kangaroo are subclasses of Animal, as in our previous lessons, Animal ed can be a Dog and then a Kangaroo.

Dynamic Binding

Method selection, or binding is done at run-time. This is when the methods are selected based on the type of the object (right side of equal sign), not the reference type (left side of equal sign).

Animal ed = new Kangaroo (); Animal fred = new Dog(); 

Both ed and fred are of type Animal, but one is a Kangaroo and the one is a Dog. They will each use their appropriate methods at run-time.

Static Binding

Let’s say that Dog has a special method, wag() that only belongs to the Dog class. That means that the Animal class does not have the wag() method. If we say:

Animal bob = new Dog (); bob.wag (); 

We will get a compile error! This is because of static binding. At compile time, the computer will look at the identifier type. In our case this is Animal. Since Animal does not have a wag() method, it will not compile.

We can avoid static binding by downcasting. Downcasting is the act of casting a reference of a superclass to a subclass. In our above example it would look like this:

Animal bob = new Dog (); (Dog) bob.wag (); 

This will tell the compiler that we want bob to be cast as a Dog so please look for the wag() method in the Dog class.

Object Superclass

The Object class is the superclass of all other classes in Java. The Object class is part of the java.lang package. The following Object class methods and constructors – including what they do and when they are used – are part of the Java Quick Reference Links to an external site.:

  • boolean equals(Object other)
  • String toString()

Subclasses of Object often override the equals and toString methods with class-specific implementations. We wrote some of our own toString methods when we designed our own classes.

Book Icon Click on "Runestone Academy" below to open the required reading that is listed.

Runestone Academy: AP CSA – Java Review Links to an external site.

  • READ: 11.15 – Polymorphism

Book Icon Click on "Introduction to Computer Science Using Java" below to open the required reading that is listed.

Introduction to Computer Science Using Java Links to an external site.:

  • READ: Chapter 82: More About Polymorphism
  • DO: Complete the quiz for practice.

Practice Icon READ: 5 Steps to a 5

  • Concept 8

Inheritance Review

Click below to complete the Inheritance Review activity.

 

Practice-It! Practice

Practice Icon Go to the PracticeIt website Links to an external site..

  • Log into account.
  • Click on Start Practicing!
  • Go to the most recent edition.
  • Click on Chapter 9: Inheritance and Interfaces
  • Complete Self-Check: 9.9
  • Click on University of Washington CSE 14(CS2) (in Practice-It menu)
  • Click on CS2 Sections
  • Click on Polymorphism
  • Complete: FirstSecondThirdFourth, HarryLarryMaryJerry, FooBarBazMumble, & FeeFieFoFum

Inheritance Test Review

  1. Go to AP Classroom Links to an external site..
  2. Complete Unit 9 Progress Check(s).

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