Object Concepts jnn

7/24/2019 Object Concepts jnn

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Basic Object-Oriented concepts


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Concept: An object has behaviors

In old style programming, you had:

data, hich as completely passive

!unctions, hich could manipulate any data

An objectcontains both data and methods

that manipulate that data

An object is active,not passive" it doesthings

An object is responsible!or its on data

But: it can exposethat data to other objects


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Concept: An object has state

An object contains both dataand methods

that manipulate that data

#he data represent the stateo! the object

$ata can also describe the relationships beteen

this object and other objects

%&ample: A CheckingAccountmight haveA balance'the internal state o! the account(

An owner'some object representing a person(


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%&ample: A )*abbit+ object

ou could 'in a game, !or e&ample( create an

object representing a rabbit

It ould have data:o hungry it is

o !rightened it is

.here it is

And methods:

eat, hide, run, dig


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Concept: Classes describe objects

%very object belongs to 'is an instanceo!( a

class

An object may have !ields, or variables

#he class describes those !ields

An object may have methods

#he class describes those methods

A class is li/e a template, or coo/ie cutter


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Concept: Classes are li/e

Abstract $ata #ypes An Abstract $ata #ype'A$#( bundles

together:

some data, representing an object or 0thing0

the operations on that data

%&ample: a CheckingAccount, ith

operations deposit, withdraw, getBalance,etc1

Classes en!orce this bundling together


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%&ample o! a class

class Employee { // fields String name; double salary; // a method oid pay !" { System#out#println!$%ay to the order of $ &

name & $ '$ & salary"; ((


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Appro&imate #erminology

instance 2 object

!ield 2 variable

method 2 !unction

sending a message to an object 2

calling a !unction

#hese are all approximatelytrue


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Concept: Classes !orm a hierarchy

Classes are arranged in a treeli/e structure called a

hierarchy

#he class at the root is named )b*ect %very class, e&cept )b*ect, has a superclass

A class may have several ancestors, up to )b*ect

.hen you de!ine a class, you speci!y its superclass I! you don3t speci!y a superclass, )b*ectis assumed

%very class may have one or more subclasses


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%&ample o! 'part o!( a hierarchy

A +ileialogis a ialogis a -indowis a Container1

Container

%anel Scroll%ane -indow

ialog +rame

+ileialog


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C44 is di!!erent

In C44 there may be more than one root

but not in 5ava6

In C44 an object may have more than one

parent 'immediate superclass(

but not in 5ava6

5ava has a single, strict hierarchy


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Concept: Objects inherit !rom

their superclasses A class describes !ields and methods

Objects o! that class have those !ields and

methods

But an object alsoinherits:

the !ields described in the class7s superclasses

the methods described in the class7s superclasses

A class is nota complete description o! its

objects6


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%&ample o! inheritance

class %erson { String name;

String age; oid birthday !" { age . age & ; (

(

class Employee e0tends %erson {

double salary; oid pay !" { ###((

%very Employeehas a name, age, and birthdaymethod as well asa salaryand a paymethod1


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Concept: Objects must be created

1 int n; does to things:it declares that n is an integer variable

it allocates space to hold a value !or n

1 Employee secretary; does onethingit declares that secretary is typeEmployee

1 secretary . new Employee ! "; allocatesthe space


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8otation: o to declare and

create objects Employee secretary; // declares secretary

secretary . new Employee !"; // allocates space

Employee secretary . new Employee!"; // both

But the secretary is still 0blan/0

secretary#name . $Adele$; // dot notation

secretary#birthday !"; // sends a message


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8otation: o to re!erence a

!ield or method Inside a class, no dots are necessary

class %erson { ### age . age & ; ###(

Outside a class, you need to say hich object

you are tal/ing to

if !*ohn#age 2 34" *ohn#birthday !";

I! you don7t have an object, you cannot use its

!ields or methods6


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Concept: thisobject

Inside a class, no dots are necessary, because

you are or/ing on thisobject

I! you ish, you can ma/e it e&plicit:

class %erson { ### this#age . this#age & ; ###(

1 thisis li/e an e&tra parameter to the method

ou usually don7t need to use this


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Concept: A variable can hold

subclass objects 9uppose B is a subclass o! A

5 Aobjects can be assigned to Avariables

5 B objects can be assigned to B variables

5 B objects can be assigned to A variables, but

5 Aobjects can notbe assigned to Bvariables

%very Bis also an Abutnot every A is a B

ou can cast: b6ariable . !B" a)b*ect;In this case, 5ava does a runtime chec/


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%&ample: Assignment o!

subclassesclass og { ### (class %oodle e0tends og { ### (og myog;og roer . new og !";%oodle your%oodle;%oodle fifi . new %oodle !";

myog . roer; // ok

your%oodle . fifi; // okmyog . fifi; //okyour%oodle . roer; // illegalyour%oodle . !%oodle" roer; //runtime check


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Concept: ethods can be

overridden

9o birds can !ly1 %&cept penguins1

class Bird e0tends Animal { oid fly !String destination" { location . destination; ((

class %enguin e0tends Bird {

oid fly !String whateer" { ((


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Concept: $on7t call !unctions,

send messages Bird someBird . pingu;

someBird#fly !$South America$";

$id pinguactually go anyhere;ou sent the message fly!###"to pingu

I! pinguis a penguin, he ignored it

otherise he used the method de!ined in Bird

ou did notdirectly call any method


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9nea/y tric/: ou can still use

overridden methods

class +amily7ember e0tends %erson { oid birthday !" { super#birthday !"; // call oerridden method gie%resent !"; // and add your new stuff (

(


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Concept: Constructors ma/e objects

%very class has a constructorto ma/e its objects


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9ynta& !or constructors

Instead o! a return type and a name, just use

the class name

ou can supply arguments

Employee !String the8ame, double theSalary" {

name . the8ame; salary . theSalary;(


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#ric/:


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Internal or/ings:

Constructor chaining I! an Employeeis a %erson, and a %ersonis an

)b*ect, then hen you say new Employee !"#he Employeeconstructor calls the %ersonconstructor

#he %ersonconstructor calls the )b*ectconstructor

#he )b*ectconstructor creates a ne )b*ect

#he %ersonconstructor adds its on stu!! to the )b*ect

#he Employeeconstructor adds its on stu!! to the%erson


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#he case o! the vanishing

constructor I! you don7t rite a constructor !or a class, 5ava

provides one 'the default constructor(

#he one 5ava provides has no arguments I! you rite anyconstructor !or a class, 5ava

does notprovide a de!ault constructor

Adding a per!ectly good constructor can brea/a constructor chain

ou may need to !i& the chain


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%&ample: Bro/en constructor

chain


%erson !String name" { this#name . name; ((class Employee e0tends %erson { double salary;

Employee ! " { // here 9aa tries to call new %erson!" but cannot find it; salary . :#4; ((


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>i&ing a bro/en constructor chain

9pecial synta&: super!###"calls the superclass constructor

.hen one constructor calls another, that call must be first

class Employee {double salary;

Employee !String name" { super!name"; // must be first

salary . :#4; ((

8o you can only create %mployees ith names

#his is !air, because you can only create ?ersons ith names


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#ric/: one constructor calling

another1 this!###"calls another constructor !or this same class

It is poor style to have the same code more than once

I! you call this!###", that call must be the first thing in yourconstructor

class Something {

Something !int 0, int y, int


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Concept: ou can control access

class %erson { public String name; priate String age; protected double salary; public oid birthday { age&&; ((

%ach object is responsible !or its on data

Access control lets an object protect its data

.e ill discuss access control shortly


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Concept: Classes themselves can

have !ields and methods


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%&ample o! a class variable


int age; static int population; %erson !String name" { this#name . name;

this#age . ; population&&; ((


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Advice: *estrict access

Alays, alwaysstrive !or a narro inter!ace

>ollo theprinciple o! in!ormation hiding:

the caller should /no as little as possible

about ho the method does its job

the method should /no little or nothing about

here or hy it is being called a/e as much as possible priate


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Advice:


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@inds o! access

5ava provides !our levels o! access:

5 public: available everyhere

5 protected: available ithin the pac/age 'in thesame subdirectory( and to all subclasses

de!ault: available ithin the pac/age

5 priate: only available ithin the class itsel! #he de!ault is calledpac/agevisibility

In small programs this isn7t important111right;


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Object Concepts jnn