3-Java-OOP

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By Waqas 1

Object OrientedProgramming

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Object Oriented

Programming

Object

Class

Variables

instance variablesclass variables

Methods

instance methods

class methods

Constructors

Encapsulation

Inheritance

Composition

Polymorphism

Abstraction

Interfaces

Nested Classes

Access Modifiers

public, private,

protected

Packages

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In object-oriented programming, a computer program may be seen as a

collection of individual units, or objects, that act on each other.

It is opposite to a traditional programming in which a program may be seen

as a collection of functions, or simply as a list of instructions to the

computer.

Each object is capable of receiving messages, processing data, and sending

messages to other objects.

Each object can be viewed as an independent little machine or actor with a

distinct role or responsibility.

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Object

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Object-oriented programming focuses on the

development of reusable software components, calledobjects.

An object is a building block

which contains variables and methods.

Objects are key to understanding object oriented

technology.

You can look around and can see many examples of

real-world objects: dog, car, table, chair, bicycle.

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All real world objects have two characteristics:

state and behavior

For example car have states

(current gear, number of gears, color, number of wheels)

and behaviors

(braking, accelerating, slowing down, changing gears)

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Software objects are modeled after real-world objects and they

also have state and behavior.

A software object maintains its state in one or more variable.

A software object implements its behavior with methods.

variables

(state)

Software Object

methods

(behavior)

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So our car object look like the following figure.

10 mph(speed)

5th gear

(currentgear)

red

(color)

Change

gear

brake

accelerate

Car Object

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Class

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In the real world, we often have many objects of the same

kind. For example, my car is just one of many cars in the

world.

Using object-oriented terminology, we can say that my car

object is an instance of the class of objects known as cars.

Cars have state (4 gears, 1 engine, 4 wheels) and behavior

(change gears, accelerate) in common. However, each car¶s

state is independent and can be different from each other.

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When car manufacturers build cars they take

advantage of the fact that cars share common

characteristics, by building many cars from the same

blueprint.

It would be very inefficient to produce a new blueprint for every individual car manufactured.

In object-oriented, it is also possible to have many

objects of the same kind that share characteristics.Classes provide the benefits of creating a template of

objects.

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we can take advantage of the fact that objects

of the same kind are similar and we can createa blueprint for those objects.

A template or blueprint of objects is called a

class.

³A class is a template or blueprint that defines the

variables and the methods common to all objects of

a certain kind.´

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Change

gear

Brake

Number

of gears

Number of wheels

Car object

After you've created the car class, you can create

any number of car objects from the class. Eachobject gets its own copy of all the variables defined

in the class.

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Brake Brake

Change

gear Change

gear

speed = 15 speed =

10

Color =

red

Color =

blue

gears = 4 gears = 4

Your Car

These two car objects created from the car class.

My Car

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Creating

Classes

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class ClassName

{

variable 1;

variable 1; method1(){}

method2(){}

}

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class Car

{

int gears;

int wheels;

public void changeGear()

{}

}

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Creating

Objects

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You can create an object of class with the

following syntax: -

ClassName objVariable = new ClassName( );

So the car class object can be created as:

Car c = new Car( );

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The object creation statement has three parts.

Car c = new Car( );

Declaration Instantiation Initialization

1. Declaration

2. Instantiation

3. Initialization

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Declaration

You declare variables of int type as: -int a;

You can say that a is a variable who can refer

to any type of int data.

Classes in java are also types so you can

declare class type variable as: -

Car c;You can say that c is a variable who can refer

to any type of Car .

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Declaring a variable do not create any object.

The code Car c; does not create a new car object, it just declare a variable named c that

will be used to refer to a Car object. The

reference is still empty until assigned with a

new keyword.The empty reference is called null reference

in java.

c

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Instantiation

The new operator instantiates a class byallocating a memory for a new object. The

new operator returns a reference to the object

it created and this reference is assigned to the

appropriate variable.

Car object

c

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Initialization

The object is always initialize by calling aconstructor.

A constructor is a special method which has a

same name as class and used to initialize the

variables of that object.

In this case the Car class object is initialized by calling the constructor of Car class Car( );

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Constructor

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Constructor

Constructor is a special method in java class.

It is used to initialize a new object variables.

A constructor has the same name as the class.

For example if you have Color class the

constructor of color class is also Color( );

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Constructors cannot return any value not even void.

If we are not providing any constructor for a class thandefault (no parameter) constructor is automatically

provided by the runtime system.

When we will create our own parameterize constructor.

We have to create our own default constructor as well

because then default constructor is not available.

The constructor can be private, public or protected.

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Java supports name overloading of

constructors so that a class can havenumber of constructors with the same

name.

The compiler will determine at run time

which constructor to call by matching the

number and type of arguments you are

passing into the constructor.

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this

Java this keyword is used to invoke anymethod or variable of current class.

The syntax of method is: -

this.methodname( );

The syntax of variable is: -

this.variablename = 7;

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If you want to invoke another constructor of

current class inside constructor you can use

this statement as follows:

this(4, 5);

The above statement will invoke the currentclass constructor that takes two int arguments.

Call to current class constructor with this

would be the first statement in theconstructor.

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Destroying

Objects(Garbage Collection)

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Some programming languages required thatyou keep track of all the objects you create and

then you explicitly destroy them when they are

no longer needed.

The Java platform allows you to create as

many objects as you want and you don't haveto worry about destroying them.

Object Destruction & Garbage Collection

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The Java runtime environment deletes objects when

it determines that they are no longer being used.

This process is called garbage collection.

An object is eligible for garbage collection whenthere are no more references to that object for

example if object goes out of scope.

You can explicitly drop an object reference bysetting the variable to the special value null.

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If you want to invoke garbage collector in the

program you can do this by issuing followingcommands.

System.gc();

Runtime.getRuntime().gc();

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finalize() method

If you want an object to clean up its state

before it is deleted from the memory, you can

declare finalize() method in the class.

This method will be called by the garbage

collector before deleting any object of this

class.

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This method is inherited from the parent

Object class in all java classes and can beoverridden like the following code:

protected void finalize(){

super.finalize();

// clean up code here.}

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Class

Variables(static variables)

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The runtime system allocates class variables once per class

regardless of the number of instances created of that class.

The system allocates memory for class variables the first

time it encounters the class at class load time.

All instances share the same copy of the class variables.

You can access class variables through an instance or through the class itself.

Class Variables

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To declare a class variable follow the

following syntax.

static Ty pe variable;

e.g.

static int a;

static double d;

If any object change the value of classvariable, the change will effect the values of

all the variables.

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Because the class variable belongs to the

class rather than any specific variable, it can

be accessed using the class name such as: -

ClassName.variableName;

Suppose you have class called Car and class

variable called gears than you can access that

variable like this: -

Car.gears;

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Class Variablevs.

Instance Variable

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All objects have their own

copy of instance variable.

All objects share the single

copy of static variable.

Instance variable is declared

without static keyword.

int a = 4;

Class variable is declared by

using static keyword.

static int a = 4;

Instance variableClass variable

Class vs. Instance Variables

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Instance variable cannot be

accessed without creating an

object.

ObjectName.variable

Static variable can be

accessed without creating an

object, by using class name.

ClassName.variable

Instance variable depends on

the object for which it isavailable.

Static variable does not

depend on the single object because it belongs to a class.

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Methods

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Method

A method is a group of programminglanguage statements with a given name.

Method syntaxReturnty pe methodname (parameters)

{

statement 1;

statement 2;«..

}

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void sum ( int a, int b )

{

statement 1

statement 2««..

}

Declaration

Body

Method has two major parts:

1. Method Declaration

2. Method Body

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Method that does not return a value

If a method does not return a value, itsreturn type must be declared void.

For example:

void abc( )

{System.out.println(³Hello World´);

}

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Method that returns a value

Methods that have a return type other than void return a value to the calling

routine using the following form of the

return statement.

return value;

int abc( ){

return 5+5;

}

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Class

Methods(static methods)

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The class (static) methods is not

referenced through a particular instance

of a class but through the class itself.

You don¶t have to create an object of the

class to invoke a static method.

Creating StaticMethods

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static Returnty pe methodname (parameters)

{

statement 1;

statement 2;

«..

}

StaticMethod Syntax

static void display(int a, int b)

{System.out.println(a+b);

}

example

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Class Methodsvs.

Instance Methods

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All objects have their own

copy of instance method.

All objects share the single

copy of static method.

instance methods are

declared without statickeyword.

Static methods are declared

by using static keyword.

InstanceMethodStaticMethod

Static vs. InstanceMethods

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Instance method cannot be

invoked without creating anobject.

ObjectName.method( );

Static method can be

invoked without creating anobject, by using class name.

ClassName.method( );

Instance method depends on

the object for which it is

available.

Static method does not

depend on the single object

because it belongs to a class.

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Instance methods can refer

to this and super.

Static methods cannot refer

to this or super.

Non-static methods can

access static variables.

Static methods cannot

access not-static variables.

Non-static methods can call

static methods.

Static methods cannot call

non-static methods.

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There are also static blocks of code in java,which have the following syntax:

static

{statements;

}

These blocks are used to initialize the

variables and execute at class load time.

Static Block s

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static

import

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static import

J2SE 5.0 introduces a new feature calledstatic import that lets you import the static

variable of a class so that you can refer to

them without having to mention the classname.

import static java.lang.Math.PI;

import static java.lang.System.out;

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Methodswith variable length

parameters

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Method with variable length parameters

J2SE 5.0 introduces a new feature that letsyou define methods with a variable number of

parameters, so that you can make several

method calls with a variable number of arguments.

These methods are called variable-lengthargument methods.

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Syntax

The variable-length parameters list consists of a type followed by three dots and the variable

name.

public void print(int... values)

{

for(int i: values)

{

System.out.println(i);}

}

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Note

There must be only one variable length parameter list in the method.

Following method is invalid

public void print(int... a, int... b)

{

}

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Note

If there are individual parameters in additionto the list, the variable length parameters list

must appear last inside the parameters of the

method.

Following method is valid:

public void print(String s, int... b){

}

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Method

Overloading

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Method Overloading

In java, it is possible to define two or moremethods with the same name, within the same

class.

They share the same name as long as their

parameters are different.

This process is known as ³methodoverloading´.

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When java call an overloaded method, it

simply executes the version of the method

whose parameters match the arguments.

Two methods are overloaded if there:

1. No of parameters are different

2.T

ype of parameters is different3. Order of parameters is different

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1. No of parameters are different

public void add(int i)

{

System.out.println(i+i);

}

public void add(int i, int j)

{System.out.println(i+j);

}

public void add(int i, int j, int k)

{

System.out.println(i+j+k);

}

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2. Type of parameters is different

public void add(int i, int j)

{

System.out.println(i+j);

}

public void add(double i, double j)

{System.out.println(i+j);

}

public void add(double i, int j)

{


}

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3. Order of parameters is different

public void add(double i, int j)

{


}

public void add(int i, double j)

{


}

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Encapsulation(Data Hiding)

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Encapsulation

Packaging an object's variables within the protectivecustody of its methods is called Encapsulation.

age

get«

set« Calling Object

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A powerful benefit of encapsulation is the hiding of

implementation details from other objects.

The internal portion (variables) of an object has

more limited visibility than the external portion

(methods).

This will protect the internal portion against

unwanted external access.

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ImplementingEncapsulation

private int age;

First step is to create the variables of the object private so that nobody

can access them from outside.

Second step is to provide public setters and getters methods for

accessing the private variables.

public void setAge(int age)

{

this.age = age;

}

public int getAge()

{

return this.age;

}

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public class Student

{

private String age;

public void setAge(int age)

{

if(age<=0)

{

System.out.println(³Invalid Age´);

}else

{

this.age = age;

}

}

public int getAge()

{

return this.age;

}

}

Implementing Business Logic with Encapsulation

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Inheritance(Deriving Classes)

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Inheritance is one of the most importantconcept in object-oriented programming, and it

has a direct effect on how you design and write

Java classes.

Inheritance is a powerful mechanism that

allows a class to inherit functionality from an

existing class.

For example, sports car, luxury car, both are

kinds of cars.

Inheritance

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Car

Sports Car Luxury Car

S ports Car and Luxury Car are sub classes or child classes of

Car class and Car class is the super or parent class of Luxury

Car and S ports Car.

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Each subclass inherit s all variables and methods

from the super class except private variables and

methods.

However, subclasses are not limited to the variables

and methods provided to them by their super class.

Subclasses can add variables and methods or can

change the definition of the existing methods

according to their own requirements.

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We use extends keyword to create child classin java as follows:

class child-class extends parent-class

{

Class bod y

«

}

Deriving Classes

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class Shape

{

}

class Oval extends Shape

{

}

class Triangle extends Shape

{

}

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The object of child class can call any non

private method from the super class that is why

we can say that:

A child class object is a parent class object.

As there is no multiple inheritance in java so In

java every class can extend only one class.However one parent class may have more then

one child classes.

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The topmost class in the java class

hierarchy is called ³Object´. If you

declare a class which does not sub

class of any super class than your

class is considered to be the child

class of the class Object.

Java Class Hierarchy


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C C i O

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When you create an object of child class thenchild class constructor first calls the parent

class default constructor before it performs its

own initialization.

The parent class constructor calls its super

class constructor and so on.

This is necessary because if the parent class isnot properly initialize how the child class

object can be created properly.

Constructor Calling Order

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If you don¶t want to invoke the default constructor

of super class but want other constructor than you

use super to invoke the parent class constructor.

super(4);

This will invoke the parent class constructor that

would take one int argument. Call to parent class

constructor with super would be the first statementin the child class constructor.

super

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You can also invoke any method or variable of

super class in the child class with the super

keyword.

super.methodname( );

super.variablename = 7;

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Method

Overriding

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Method Overriding

When a child class defines a method with

the same name and signature as the parent,

then it is said that the child¶s version

overrides the parent¶s version in his favor.

When an overridden method is called from

child class object, the version in the child

class is called not the parent class version.

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The return type, method name, number and

type of the parameters of overridden method

in the child class must match with themethod in the super class.

You can call the super class method fromthe child class with super keyword.

super.overriddenMethodName( );

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Final

Keyword

final keyword

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final keyword

The keyword final has three usages:

1. Final variables.

2. Final methods.

3. Final classes.

1. Final variables

Final modifier used with the variables specify that

the variable has a constant value and it can not

changed.

final int age = 10;

2 Fi l th d

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2. Final methods

When we use the keyword final with methodsthen it specifies that the method cannot be

overridden in the child class.

final void volume( )

{

}

3 Fi l l

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3. Final classes

The keyword final can be applied to classes.If this is done, the class cannot be inherited.

This provides security features and stops

further extensions of the class.

final class City

{

}

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Composition

Composition

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Composition

Composition is the ability of composing

other objects inside classes.

Car Object

Engine Object

doubleweight;

String

make;

String color

int wheels

Engine e;

public class Car

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public class Car

{

String color;

int wheels;

Engine e; // Single Object

Gear g[]; // Multiple Objects

}

public class Engine{

double weight;

String make;

}

public class Gear

{

««

}

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Polymorphism(Late Binding or Dynamic Binding)

Polymorphism

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Polymorphism is the ability to assume different

forms.

In object-oriented programming, this refers to the

ability of objects to have many methods with differentimplementations.

In Polymorphism program does not need to know theexact type of object and method in advance.

Polymorphism

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Caller

A

B

C

Exchange decide at call time to which

department it may transfer the callaccording to the request of the caller.

Polymorphism is a phenomena in which the parent

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Polymorphism is a phenomena in which the parent

class object call the appropriate overridden method of

child class at run time.

Shape

public void

draw(){

}Oval

public void draw()

{

}

Rectangle

public void draw()

{

}

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Abstraction

Abstraction

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Abstraction

Abstraction is a design technique that focuses on the essentialaspects of an entity and ignores or conceals less important or

non-essential aspects.

A process of identifying which details in a given context areessential, and should be visible, and which are non-essential

and can be hidden "behind the scenes".

Abstraction is an important tool for simplifying a complex

situation

Abstract Classes

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Abstract Classes

An abstract class is a class that is partiallyimplemented and whose purpose is solely

represent abstract concept.

Abstract classes are made up of one or more

abstract methods.

Abstract classes cannot be instantiated becausethey represent abstract concept.

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For example: Food represents the abstract concept

of things that we can eat, that is why we have never

seen the instance of food what we see instead are

instances of burger, apple, chocolate.

In Java classes such as Number represents the

abstract concept of numbers therefore it not be

instantiated.

What we instantiate the subclasses of Number such

as Float, Integer etc.

Abstract Class Declaration

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public abstract class Food

{

}

Abstract Class Declaration

As Food is abstract class so the following statement will not

compile.

Food f = new Food();

AbstractMethods

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AbstractMethods

An abstract method is a method which has no

implementation (body).

The body of this method is provided by a subclass of

the class in which the abstract method is declared.

The syntax of abstract method is:

public abstract void draw( );

Abstract Classes &Methods Concepts

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Abstract Classes &Methods Concepts

Abstract class can not be instantiated it meansthat you can not create object of abstract class

with new keyword.

Abstract class can only be sub classed.

A class derived from abstract class must override

all of the parent class¶s abstract methods. If it

does not define all methods then it will declareitself as abstract.

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An abstract class can inter mix abstract and non

abstract methods.

Abstract class and method cannot be declared as

final.

abstract method cannot be declared as static.

Abstract method cannot be declared as private.

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Interface

Interface

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An interface defines a protocol of behavior that can be implemented by any class anywhere in the class

hierarchy either they are related or not.

An interface defines a set of methods but does notimplement them.

A class that implements the interface agrees to

implement all the methods defined in the interface.

Hierarchy A Hierarchy B

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Interface

Interface is a bridge between two related or unrelated classes

so that they can share common data and implementations and

communicate with each other.

Declaring Interface

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g

public interface Database{

public final static int a = 4;

public final static int b = 5;

public abstract void insert( ); public abstract void update( );

}

All variables are public final and static by default

All methods are public abstract by default

Implementing Interface

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T

he above class must need to define all the methods in theinterface Data and Driver, otherwise it declares itself

abstract.

public class Oracle implements Database

{

public abstract void insert( )

{}

public abstract void insert( ){}

}

ImplementingMultiple Interfaces

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public class Oracle implements Database,

Connection, Command {

}

p e e g u p e e ces

Class can implement more than one interface.

Now this class has to define all the abstract methods of Database, Connection and Command interfaces.

Interfaces Inheritance

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An interface can extend another interface.

public interface Database

{

}

public interface M ySq l extends Database

{

}

Interfaces andMultiple Inheritance

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An interface can extends any number of interfaces.

p

public interface Database

{

} public interface Connection

{

}

public interface M ySq l extends Database,

Connection

{

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Interface

vs.

Abstract Class

Because an interface is simply a list of unimplemented, and

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Because an interface is simply a list of unimplemented, and

therefore abstract, methods, you might wonder how an interface

differs from an abstract class.

An abstract class can define some

methods.

An interface cannot define any

method.

Abstract class is declared with theclass keyword.

Interface is declared with theinterface keyword.

Abstract ClassInterface

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Abstract classes are part of the

class hierarchy.

Interfaces are not part of the class

hierarchy.

We can not extend more then one

abstract class.

We can implement multiple

interfaces on a single java class.

We extends abstract classes using

extends keyword.

We implement interface on class

using implements keyword.

Abstract ClassInterface

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Nested

Classes

Nested Classes

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Nested Classes

Java lets you define a class as a member of

another class. These classes are called nested

classes.

As a member of its enclosing class, a nestedclass has a special privilege:

It has unlimited access to its enclosing class's

members, even if they are declared private.

public class Shape

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Like static variables and methods, a nested class can

also be declared static.

A static nested is called just a static nested class.

A non-static nested class is called inner class.

{

public class Point

{

}

}

Creating Object of Nested Class

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g j

public class Shape

{

Point p = new Point();

public class Point

{

Shape s = new Shape();

}

}

Inner class

object

Outer class

object

Creating Object of Nested Class in Third Class

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public class Shape

{ public class Point

{

}

}

Inner class

ob ject

public class Test

{

public static void main(String args[])

{Shape.Point p = new Shape().new

Point();

}

}

Extending Nested Class through Inheritance

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public class Shape{

public class Point

{

}

} public class Oval extends Shape

{

public class CenterPoint extendsShape.Point

{

}

}

Some Facts about Nested Classes

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1. Like static methods static nested class cannot directlyaccess instance methods and variables.

1. Nested classes can be declared abstract.

1. Nested classes can be declared final.

1. Nested classes can also be declared private, public,

protected or default just as other member methods.

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Access

Modifiers

Access Modifiers

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Access modifiers define various levels of access between

class members and the outside world (other objects).

They allow us to define the encapsulation characteristics of

an object.

There are four access modifiers in java:

1. private

2 . protected

3 . public

4. default.

private

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The private access modifier is the most

restrictive; it specifies that class members areaccessible only by the class in which they are

defined. This means that no other class has

access to private class members, evensubclasses.

private int a = 4;

private void show ( )

{

}

BusCar Burg

erPizza

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private variable a is only accessible in Car

class.

Bus

Package 1 Package 2

Carprivate int a;

BurgerPizza

protected

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This modifier specifies that class members

are accessible only to methods in that class,the classes inside the package and subclasses

of that class outside the package.

protected int a = 4;

protected void show ( )

{

}

BusCar Burg

erPizza

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protected variable a is accessible inside Car

class and inside child class which can be insame package or different package.

Bus

Package 1 Package 2

CarProtected int a;

BurgerPizza

public

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The public access modifier specifies that

class variables and methods are accessible toanyone, both inside and outside the class.

This means that public class members have

global visibility and can be accessed by anyother object.

public int a = 4;

public void show ( )

{

}

BusCar Burg

erPizza

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public variable a is accessible from all 4

classes in any package.

Bus

Package 1 Package 2

Carpublic int a;

BurgerPizza

DefaultAccess Modifier

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The default (package) access modifier

specifies that only classes in the same packagecan have access to a class's variables and

methods.

There is no actual keyword for declaring the

default access modifier.

it is applied by default in the absence of anyaccess modifier.

BusCar Burg

erPizza

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Int variable a is accessible only inside Package

1.

Package 1 Package 2

int a;

g

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Packages

Packages

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A package is a collection of related classes and

interfaces.

Packages provide following benefits:

Classes are easier to find and use No naming conflictions

Control access for class members

For example you have classes graphics objects likeCircle, Square, Rectangle etc.

You should bundle these classes into package for the

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You should bundle these classes into package for the

following reasons:

1. You are other programmers can easily determine

that these classes are related.

1. You and other programmers know that where to

find classes that provide graphics related functions.

1. The name of your class wouldn¶t conflict with class

names in other packages.

Creating Packages

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To create a package we put the package

statement at the top of the source file in

which the classes and interfaces are defined.

package graphics;

public class Circle

{

}

To put Rectangle the class inside graphics package,

the following code appears inside the Rectangle java

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the following code appears inside the Rectangle.java

file.

Now both Circle and Rectangle classes are in the

graphics package.

package graphics;

public class Rectangle

{

}

if you want Circle class to available outside

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if you want Circle class to available outside

the package. You must use the class fullyqualified name as follows:

graphics.Circle c = new graphics.Circle( );

This makes your code difficult to read.

import Statement

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Java provide a solution to limit the use of fully qualified name

each time you create an object.

You can just import the class you need with import statement.

To import a specific member into the current file, put an import

statement at the beginning of your file before any class or

interface definitions but after the package statement, if there is

one.

Here's how you would import the Circle class from thegraphics package

import graphics.Circle;

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Now you can refer to Circle class by its simple name:

Circle c = new Circle( ):

To import all the classes in the graphics package you

can use * as wild card character as follows:

import graphics.*;

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WaqasAnwar

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