object-oriented programming inheritance · 2010. 2. 22. · object-oriented programming inheritance...

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Object-Oriented ProgrammingInheritance

Ewan Klein

School of Informatics

Inf1 :: 2009/10

Ewan Klein (School of Informatics) Object-Oriented ProgrammingInheritance Inf1 :: 2009/10 1 / 30

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Admin

Coming attractions

Week 7: More revision sessions at 1500 and 1600 this coming Friday (AT5.0)

Week 8: Revision sessions can be scheduled if there’s a demand

Week 9: Mock Exam; no tutorials, no labs, (yes lecture)

Week 10: OOP Competition (interactive computer game), judging onFriday 19th March


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Classes with Stuff in Common

takeExam()graduate()party()

matricNonameagemailBox

UG Student PG Student

takeExam()graduate()party()tutor()


UML diagram: class name, instance variables, methods.


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Classes with Stuff in Common

takeExam()graduate()party()


UG Student PG Student

takeExam()graduate()party()tutor()



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Abstracting Common Stuff

graduate()//get BSc

UG PGgraduate()//get PhDtutor()

takeExam()party()


Student


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Abstracting Common Stuff

Subclass (e.g. UG) inherits from superclass (e.g. Student).

In Java: subclass extends superclass.

So subclass inherits all the members — instance variables and methods — ofsuperclass.

A subclass can add new members of its own

By default, methods that are inherited from superclass have sameimplementation in subclass; except

if subclass overrides the inherited methods.


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Doctor Example, 1

+treatPatient(): void+makeIncisions(): void

Surgeon

+giveAdvice(): void

FamilyDoctor

+makesHouseCalls

+treatPatient(): void

+worksAtHospital

Doctor

generalization

overriding method

For handy guide to UML, seehttp://www.loufranco.com/blog/assets/cheatsheet.pdf


http://www.loufranco.com/blog/assets/cheatsheet.pdf

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Doctor Example, 2

.Doctor

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public class Doctor {public boolean worksAtHospital;public void treatPatient() {

// perform a checkup}

}


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Doctor Example, 3

.Surgeon..

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public class Surgeon extends Doctor {public void treatPatient() {

// perform surgery// overrides inherited method

}public void makeIncisions() {

// use a scalpel}

}

NB We put this class into a new file Surgeon.java


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Doctor Example, 4

.FamilyDoctor..

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public class FamilyDoctor extends Doctor {public boolean makesHouseCalls;public void giveAdvice() {

// tells you to wrap up warmly}

}


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Method Overriding

Method m in subclass B overrides method m’ in super class A if m has exactlythe same signature (i.e. name and parameters) and return type as m’.

In effect, m replaces the implementation of m’.

If s is an object of class Surgeon, then calling s.treatPatient() uses thesubclass implementation of treatPatient().

NB The story about return types is slightly more complicated.


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Type Hierarchy View in Eclipse

Hierarchy Tree Pane

Members Pane


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Doctor Subclasses

Right-click on a class name and select Open Type Hierarchy


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FamilyDoctor Members

Inherited and non-inherited members


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The Design Process

...1 Look for objects that have common attributes and behaviours.

...2 Design a class that represents the common state and behaviour.

...3 Decide if a subclass needs method implementations that are specific to thatparticular subclass type.

...4 Carry out further abstraction by looking for groups of subclasses that mighthave common behaviours.


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X IS-A Y?

ClassX extends ClassYCan we say that ClassX IS-A ClassY?

That is, an instance of ClassX can do anything (and maybe more) that aninstance of ClassY can do?

.IS-A Candidates..

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...1 Kitchen extends Room

...2 Room extends House

...3 Violinist extends Musician

...4 Sink extends Kitchen

...5 Musician extends Person

...6 Madonna extends Singer

...7 Soprano extends Musician


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Doctor Example in Eclipse

Where does Doctor belong in the class hierarchy?


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The class Object

.Doctor’s Superclass

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public class Doctor {

extends Object {

boolean worksAtHospital;void treatPatient() {


}

Object is the superclass of every class in Java!

If a class doesn’t explicitly extend some superclass, then it implicitly extendsObject.


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The class Object

.Doctor’s Superclass

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public class Doctor

{

extends Object {boolean worksAtHospital;void treatPatient() {


}

Object is the superclass of every class in Java!

If a class doesn’t explicitly extend some superclass, then it implicitly extendsObject.


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Object as a Polymorphic Type

.Javadoc for ArrayList’s contains() method..

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public boolean contains(Object elem)Returns true if this list contains the specified element.

Parameters: elem - element whose presence in this List is to be tested.

Returns: true if the specified element is present; false otherwise.

Since every class in Java is a subclass of Object, the contains() method cantake any object as an argument.


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FamilyDoctor Members

Inherited and non-inherited members


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Some Methods of Object

Object defines methods that are available to every class. E.g.,

equals(Object o) — test whether two objects are equal.

hashCode() — numerical ID; equal objects must have equal hash codes.

toString() — returns a textual representation of an object; automaticallyinvoked by methods like System.out.println().

Sometimes the default implementation of toString() is not what we want, souseful to override it.


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Overriding toString(), 1

Create an arbitrary (empty) class:.Number Two..

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public class NumberTwo {}

What happens if we supply a NumberTwo object n2 as the argument ofSystem.out.print()?

Invokes n2.toString(), which is inherited from Object.

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.NumberTwo@10d448

Prints the class Name, @, and the object’s hashcode concatenated into a string.


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The Prisoner (1967): I am not a number!

Patrick McGoohan

http://www.youtube.com/watch?v=29JewlGsYxs


http://www.youtube.com/watch?v=29JewlGsYxs

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Let’s override the inherited toString() method.Number Six..

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public class NumberSix {public String toString() {

return ”I am not a number – I am a free man!”;}

}


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.PrisonerLauncher

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public class PrisonerLauncher {public static void main(String[] args) {

NumberTwo n2 = new NumberTwo();NumberSix n6 = new NumberSix();System.out.println(”You are Number Two: ” + n2);System.out.println(”You are Number Six: ” + n6);

}}

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You are Number Two: NumberTwo@10d448You are Number Six: I am not a number – I am a free man!


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Printing out Arrays.PrintArray..

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import java.util.ArrayList;import java.util.Arrays;public class PrintArray {

public static void main(String[] args) {int [] numbers1 = { 33, 5, 22, 0, -3, 700 } ;ArrayList<Integer> numbers2 = new ArrayList<Integer>();for (int i : numbers1) {

numbers2.add(i+10);}

System.out.println(”Using the array:”̑ + numbers1);System.out.println(”Using Arrays.toString:”̑

+ Arrays.toString(numbers1));System.out.println(”Using the ArrayList:”̑ + numbers2);

}}


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Printing out Arrays

.Output..

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Using the array: [I@72e3b895Using Arrays.toString: [33, 5, 22, 0, -3, 700]Using the ArrayList: [43, 15, 32, 10, 7, 710]


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Summary

Implementing an interface: the implementing class must provide bodies forall the methods declared in the interface.Inheriting from a superclass:

I the subclass gets all the members (instance variables and methods) of thesuperclass;

I the subclass may add members, and also override methods.I So subclass extends (adds to) the behaviour of its superclass.

Inheritance corresponds roughly to taxonomic relations for everyday concepts— but these intuitions not always relevant for software development.


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When to use What

Use inheritance when you want the subclass to be able to use the methodimplementations of its superclass.

Use an interface when you want your classes to conform to an API, but eachclass has its own implementation of all the interface methods.


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Reading, etc

Read Chapter 7 of Head First Java.

More on Inheritance next Monday.


object-oriented programming inheritance · 2010. 2. 22. · object-oriented programming inheritance...

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