Birds and Class Hierarchy: Object-Oriented Programming with Inheritance and Polymorphism, Slides of Java Programming

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Inheritance and OOP

Chapter 11

Chapter Contents

Chapter Objectives

11.1 Introductory Example: A Trip to the

Aviary

11.2 Inheritance and Polymorphism

11.3 Example: Geological Classification

11.4 Example: An O-O Payroll Program

11.5 Graphical/Internet Java: A Function

Plotter

Objects

 Organized into groups with similar

characteristics

 they are said to be related by a common characteristic

 Object-Oriented programming seeks to

provide mechanisms for modeling these

relationships

 this is where the Java word extends is used

11.1 Intro Example: A Trip to the Aviary

 Consider a collection of birds which have

different properties

 name  color (some of the same name are of different colors)  they eat different things  they make different noises  some make multiple kinds of sounds

We seek a program to simulate this collection

Hierarchy Bird

call:? color:? food:? movement:?

WalkingBird call:? color:? food:? movement:walked

FlyingBird call:? color:? food:? movement:flew

Goose call: honk color: gray food: bugs

Ostrich call: neek-neek color: brown food: grass

Parrot call: Squawk color:? food: fruit

Owl call:? color:? food:mice

TalkingParrot

...

Coding

 Note Bird class, Figure 11.

 Attributes common to all birds

 color  food  movement

Subclasses

// FlyingBird provides subclass of Bird

abstract class FlyingBird extends Bird { public FlyingBird (String color, String food) { super (color, food, "flying"); } }

Values passed to Bird class constructor where used to initialize attribute variables defined in class Bird

Subclasses

// Parrot provides subclass of FlyingBird class Parrot extends FlyingBird { public Parrot(String color) { super(color, "fruit"); } public String getCall() { return "Squawk!"; } }

Note "is a" relationship: a parrot is a flying bird. Similarly, a Parrot is a Bird

Movement attribute not an argument for constructor, a parrot is already specified as a flying bird

11.2 Inheritance and Polymorphism

 Declaring subclasses

class B extends A {... }  means class B is a specialization of class A  the "is a" relationship exists  a B object is an A object

A

B

"is a"

increasingly general

increasingly specialized

Superclass

Subclass

Inheritance

 Other names:

 superclass also called "parent class"  subclass also called "child class"

 These names help understand concept of

inheritance

 Child class inherits characteristics of

parent class

 attributes  methods docsity.com

Results of Inheritance

 Used to eliminate redundant coding

 When we send toString() message to

a Goose or Parrot or TalkingParrot

object

 none of these classes implement the

toString() method

 but … they inherit it from Bird

 toString() need not be redefined in the

subclasses docsity.com

Handles and extends

 Consider the declaration:

Bird abird = new Goose();

 this is legal  a Goose object "is a" Bird object

 Contrast

Goose aGoose = new Bird("gray",

"walking", "bugs");

 this is NOT legal  A Bird object is not necessarily a Goose object

extends is unidirectional. A extends B does NOT imply that B extends A

Polymorphism

 A method defined in a class is inherited by all descendants of that class

 When a message is sent to an object to use method m() , any messages that m() sends will also be sent to the same object

 If the object receiving a message does not have a definition of the method requested, an inherited definition is invoked

 If the object receiving a message has a definition of the requested method, that definition is invoked

Principles:

Java Hierarchy

 The classes we have been using

throughout the whole text are organized

into a hierarchy

 Object is the common ancestor

 every class inherits characteristics of this class  for example: clone() , equals() , getClass() and toString()

 Every class must fit within the Java class

hierarchy