Object-Oriented Programming Concepts: Classes, Objects, and Access Modifiers, Exercises of Java Programming

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3-Jul-

Basic Object-Oriented Concepts

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Concept: An object has behaviors



In old style programming, you had:^ 

data, which was completely passive ^

functions, which could manipulate any data



An object contains both data and methods thatmanipulate that data^ 

An object is

active,

not passive; it

does

things

^

An object is

responsible

for its own data

^

But: it can

expose

that data to other objects

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Example: A “Rabbit” object



You could (in a game, for example) create an objectrepresenting a rabbit 

It would have data:^ 

How hungry it is ^

How frightened it is ^

Where it is



And methods:^ 

eat, hide, run, dig

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Concept: Classes describe objects



Every object belongs to (is an instance of) a class 

An object may have fields, or variables^ 

The class describes those fields



An object may have methods^ 

The class describes those methods



A class is like a template, or cookie cutter^ 

You use the class’s constructor to make objects

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Example of a class class Employee {// Fieldsprivate String name;

//Can get but not change

private double salary;

// Cannot get or set

// ConstructorEmployee(String n, double s) {

name = n; salary = s; } // Methodsvoid pay () {

System.out.println("Pay to the order of " +

name + " $" + salary);

} public String getName() { return name; } // getter }

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Approximate Terminology



instance = object 

field = instance variable 

method = function 

sending a message to an object =

calling a function



These are all

approximately

true

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Example of (part of) a hierarchy

A

FileDialog

is a

Dialog

is a

Window

is a

Container

Container

Panel

ScrollPane

Window

Dialog

Frame

FileDialog

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C++ is different



In C++ there may be more than one root^ 

but not in Java!



In C++ an object may have more than one parent(immediate superclass)^ 

but not in Java!



Java has a single, strict hierarchy

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Example of inheritance

class Person {

String name;int age;void birthday () {

age = age + 1; } }

class Employee

extends Person {

double salary;void pay () { ...} }

Every

Employee

has

name

and

age

fields and

birthday

method

as well as

a

salary

field and a

pay

method.

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Concept: Objects must be created



int n;

does two things:

^

It declares that

n

is an integer variable

^

It allocates space to hold a value for

n

^

For a primitive, this is all that is needed



Employee secretary;

also does two things

^

It declares that

secretary

is type

Employee

^

It allocates space to hold a

reference

to an Employee

^

For an object, this is

not

all that is needed



secretary = new Employee ( );^ 

This allocate space to hold a

value

for the Employee

^

Until you do this, the Employee is

null

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Notation: How to reference a field or method



Inside a class, no dots are necessary

class Person { ... age = age + 1; ...}



Outside a class, you need to say which object you aretalking to

if (john.age < 75) john.birthday ();



If you don't have an object, you cannot use its fieldsor methods!

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Concept:

this

object



Inside a class, no dots are necessary, because^ 

you are working on

this

object



If you wish, you can make it explicit:

class Person { ... this.age = this.age + 1; ...}



this

is like an extra parameter to the method



You usually don't need to use

this

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Example: Assignment of subclasses

class Dog { ... }class Poodle extends Dog { ... }Dog myDog;Dog rover = new Dog ();Poodle yourPoodle;Poodle fifi = new Poodle ();myDog = rover;

// ok

yourPoodle = fifi;

// ok

myDog = fifi;

//ok

yourPoodle = rover;

// illegal

yourPoodle = (Poodle) rover;

//runtime check

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Concept: Methods can be overridden



class Bird extends Animal {So birds can fly. Except penguins.

void fly (String destination) {

location = destination; } } class Penguin extends Bird {

void fly (String whatever) { } }

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