CIS 234: Inheritance

Dr. Ralph D. WestfallApril, 2010

What Is Inheritance? getting something from somewhere

else analogy with biological inheritance

inherit some characteristics from parents height, eye color, etc.

characteristics may be inherited from common ancestors

all breeds of dogs originally came from wolves in East Asia?

OO Programming Inheritance new class can inherit from existing

class properties (variables) methods (actions)

can add additional properties and methods to the new class

another new class can inherit from the new class that inherited from the previous class, etc. (see p. 227)

Instantiation vs. Inheritance instantiation = creating an object from

an existing class each new object has all the properties and

methods from the class properties = defaults or assigned values

inheritance = creating a new class from an existing class has all previous properties and methods can add more properties and methods to

it can instantiate objects from this new class

Inheritance Example Wolf class code

"base class" from which other code is derived

also called "superclass" or "parent class"

Dog class inherits from Wolf class "derived" class also called "subclass" or "child class"

Review What is inheritance?

What does a class get from a class it inherits from?

What does instantiation mean in terms of classes and objects?

Can a class inherit from a class that inherits from another class? If so, how many times?

Is-A (subclass) Relationship a subclass is more specific than

parent Employee is general (all people

working for the company) EmployeeWithTerritory is specific (a

certain type of employee) is-a relationship

EmployeeWithTerritory is-a Employee Chihuahua is-a Dog

Has-A (attribute) Relationship a class can have 2nd class as attribute

object within a class Car class has-a object of type

Transmissionint price; Tranny tran;Public class Car(int price, Tranny tran) { this.price = price;

this.tran = tran;}

Advantages of Inheritance key concept: inheritance makes code

reusable don't "reinvent the wheel" saves time: add what you need rather

than writing a whole class fewer errors: the other class has already

been fully debugged (hopefully) more understandable code: only need to

look at added code (rest is a "black box")

Extending Classes put keyword extends in class headerpublic class ConvertCurrency

extends ConvertToUSDollars can use methods from parent classpublic double net(String curr, double

amt){ return 1.019 * calculate(curr, amt);} // calculate is in parent class

Using Superclass Methods create a new class that extends an

existing class can write a new program that:

instantiates an object from the new class has this object call methods that are in its

superclass or just calls a class (not using as an

object) method that is in parent class' methods

Using Superclass Methods - 2 //create a class inheriting from

anotherpublic class NextClass extends FirstClass.....//code below is in a different classNextClass myObj = new NextClass();

myObj.getData(); // getData can be method in parent or child

Using Superclass Methods - 3 child class can use word super

instead of an object name to call methods in its superclass

super.toString(); //runs toString method in parent

Inheritance Is One-Way subclass can use superclass

methods and properties keyword extends makes all

superclass methods known to subclasses

superclass can't use subclass methods and properties nothing tells superclass which

subclasses are derived from it subclass methods are not "visible" to

superclass

Overriding Superclass Methods some superclass methods may not be

appropriate for subclass Employee class calculatePay method

provides overtime after 40 hours/week programmers don't get overtime

make subclass: ProgrammerEmployee add calculatePay method // no overtime new method overrides (replaces) other

Overriding Polymorphism poly means many

polytechnic = many technologies morph means shape or form

Dr. Jeckyll morphed into Mr. Hyde; image, videos; photo morphing web site

polymorphism = many shapes in OO programming, means somewhat

different ways of doing the same general thing

Polymorphism Generality create a general superclass

e.g., GeometricFigure add a general method

e.g., calculateArea as height x width create subclasses

Rectangle, Square & Parallelogram can use calculateArea method from superclass

Triangle has different code for this method

Class Diagram shows from what classes other

classes in Java inherit from subclasses have same methods as their

superclasses (if not overridden) diagrams of inheritance

Java has thousands of classes most of these inherit from other classes awt diagram

Web Links on Inheritance links found with Google, searching

on: "oo programming" "+what +is

inheritance" Sun Microsystems tutorial "Don't Fear the OOP" (cutesy)

tutorial slide 5 tutorial slide 6

Exercise identify subclasses of a superclass

Food, MusicalInstrument, MotorVehicle, Pet, or something else

identify a general method that will work for subclasses derived from superclass e.g., cookFood: boil for 10 minutes

identify subclasses & override methods e.g., Hamburger.cookFood: fry 4 minutes

Review What is the difference between is-a

and has-a? (give examples) Name some advantages of

inheritance Where is the keyword extends

used in relation to inheritance? super?

More Review What does overriding mean? What is polymorphism?

Which one was the "good guy" – Dr. Jekyll or Mr. Hyde?