object orineted programming

8/7/2019 Object Orineted Programming

http://slidepdf.com/reader/full/object-orineted-programming 1/37

Recap of day 1

y Object Oriented Programmingy

Classy Abstraction

y Encapsulationy Polymorphismy

OO Analysisy Access Modifiersy Creating Objectsy this Reference



ConstructorsConstructorsCommunication Team



Constructors (1/5)Constructors (1/5)

y A constructor is a special method that is

called to create a new object

y A constructor method

will have the same name as that of the class will not have any return type, not even void

PolicyHolder policyHolder = new PolicyHolder();

Calling theconstructor




y The coder can write a constructor in a class, if required

y If a user defined constructor is available, it iscalled just after the memory is allocated for theobject

y If no user defined constructor is provided for aclass, the implicit constructor initializes the

member variables to its default values numeric data types are set to 0 char data types are set to null character(µ\0¶) boolean data types are set to false

reference variables are set to null




public class PolicyHolder{

//Data Members

public PolicyHolder(){

bonus = 100;

}

//Other Methods}

UserdefinedConstructor

PolicyHolder policyHolder = new PolicyHolder();//policyHolder.bonus is initialized to 100



Method Overloading (1/3)Method Overloading (1/3)

y Two or more methods in a Java class can

have the same name, if their argumentlists are different

y Argument list could differ in No of parameters

Data type of parameters Sequence of parameters

y This feature is known as MethodOverloading




y Calls to overloaded methods will be resolved duringcompile time Static Polymorphism

void print(int i){

System.out.println(i);

}void print(double d){

System.out.println(d);

}

void print(char c){

System.out.println(c);

}




void add (int a, int b)

void add (int a, float b)

void add (float a, int b)

void add (int a, int b, float c)

v

oid add (int a, float b)int add (int a, float b)

Overloadingmethod

Not Overloading

Compile error



Overloading the ConstructorsOverloading the Constructors

Just like other methods, constructors also

can be overloaded

The constructor without any parameter is

called a default constructor




public class PolicyHolder{//Data Memberspublic PolicyHolder(){

bonus = 100;}public PolicyHolder(int policyNo, double

bonus){this.policyNo = policyNo;this.bonus = bonus;

}//Other Methods

}

y PolicyHolder policyHolder1 = new PolicyHolder();

y //policyHolder1.policyNo is 0 and bonus is 100

y PolicyHolder policyHolder = new PolicyHolder(1, 200);y //policyHolder1.policyNo is 1 and bonus is 200



Memory Allocation (2/2)Memory Allocation (2/2)

public class PolicyHolder{private int policyNo;private double bonus;//Other Data Memberspublic void sample(int x){

int y;//More Statements

}//More Methods

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

PolicyHolder policyHolder;policyHolder = new PolicyHolder();

}}

Data members of theclass are stored in theheap along with theobject. Their lifetimedepends on the lifetime of

the object

Local variables x and yare stored in the Stack

Local variablepolicyHolder is storedin the Stack

Dynamic objects will bestored in the heap



Lifetime of objects (1 of 2)Lifetime of objects (1 of 2)

y Policy policy1 = new Policy();

y

Policy policy2 = new Policy();y The two Policy objects are now living

y on the heap References: 2

Objects: 2y Policy policy3 = policy2;

References: 3

Objects: 2

2policyNo

bonus

1

policyNobonus

heap

policy1

policy2

2policyNobonus

1

policyNobonus

policy1

policy2

policy3

heap



Lifetime of objects (2 of 2)Lifetime of objects (2 of 2)

y policy3 = policy1;

References: 3

Objects: 2y policy2 = null;

Active References: 2

null references: 1

Reachable Objects: 1

Abandoned objects: 1

2policyNo

bonus

1policyNo

bonus

heap

policy1

policy2

policy3

2policyNo

bonus

1policyNo

bonus

policy1

policy2

policy3

heap

Null

reference

This object can begarbage collected

(Can be Removedfrom memory)



Garbage Collection

y In programming languages like C and C++, theprogrammer has to de-allocate all dynamic memory

y An object that is not referred by any reference variable willbe removed from the memory by the garbage collector

Automatic Garbage Collection

If a reference variable is declared within a function, thereference is invalidated soon as the function call ends

Programmer can explicitly set the reference variable tonull to indicate that the referred object is no longer inuse

y Primitive types are not objects and they cannot beassigned null



The static keyword

y The static keyword can be used in 3

scenarios: For class variables

For methods

For a block of code



Static Data Members

y Static data is a data that is common to the entire class

y Assume that the class PolicyHolder wants to keep track of the

total number of Policy Holders The data common to the entire class

An int data member total should be declared as static

A single copy of total will be shared by all instances of the class

public class PolicyHolder{ private int policyNo;

private double bonus;

private static int total;

//Other Data Members and Methods

}

The static variable is initialized to 0,

ONLY when the class is first loaded,NOT each time a new instance is made



Static Methods (1/3)

y Static method is a method that is

common to the entire classy Consider a method getTotal() in the class

PolicyHolder that returns the value of thestatic data member total

It is more logical to think of this method as amethod of the entire class rather than that of an object

The method getTotal() is declared as static




public class PolicyHolder{

private int policyNo;

private double bonus;

private static int total;

//Other Data Members and Methods

public PolicyHolder(){

++total;

//Other Statements

}

public static int getTotal(){

return total;}

}

Each time the constructor isinvoked and an object gets created,

the static variable total will be

incremented thus keeping a count

of the total no of PolicyHolder

objects created




y Static methods are invoked using the syntax <ClassName.MethodName>

y Static methods can access only other static data and methods

System.out.println(PolicyHolder.getTotal());

//Prints 0

//A static method can be invoked without creating an object

PolicyHolder policyHolder1 = new PolicyHolder ();

System.out.println(PolicyHolder.getTotal());//Prints 1

PolicyHolder policyHolder2 = new PolicyHolder ();

System.out.println(PolicyHolder.getTotal());

//Prints 2



The Static Block

y The static block is a block of statement inside a Java class thatwill be executed when a class is first loaded

A static block helps to initialize the static data members just like constructors help to initialize instance members

class Test{

static{

//Code goes here

}

}



Strings in Java (1/3)

y String is a system defined class in Java

y

Declaring ³Hello World´ in code will createan object of type string with data ³HelloWorld´ and returns a reference to it.

System.out.println(³Hello World´);



Strings in Java (2/3)

y A String object can be created without the new operator

Declaring ³Hello World´ in code will create an object of typestring with data ³Hello World´ and returns a reference to it.

String s = ³Java´;

String s1 = ³Hello´;

String s2 = ³World´;

String s3 = s1 + s2; //s3 will contain ³HelloWorld´

int a = 20;

System.out.println(³The value of a is ´ + a);



Command Line Arguments

y The main method takes an array of String as theparameter

y This array contains the command line arguments that arepassed when the program is invoked

> java Sample Hello Welcome Ok

Hello, Welcome and Ok will be passed as an array of 3

elements to the main method of the class Sample

class Sample{

public static void main(String [] args){

for(int i = 0; i < args.length; ++i)

System.out.println(args[i]);

}

}



A Complete Java Program - Revisited

y

The main method is public so that it can be accessed outside the class is static so that it can be invoked without creating any

objects is void and does not return anything can take command line arguments as an array of String

public class HelloWorld{

public static void main(String [] args){

System.out.println(³Hello World!´);

}

}



Can you answer these questions?

y What is a this reference?

y

What are Constructors?y What is method overloading?

y What is Automatic Garbage Collection inJava?

y What are the uses of the keyword static?



Inheritance (1/3)

y Assume that we require a class TermInsurancePolicy

Needs all the features of the class Policy

TermInsurancePolicy will have a pre defined number of years before they expire

Needs to add an extra data called term and relevantmethods

The keyword extends is used in Java to inherit a subclass from a super class

public class TermInsurancePolicy extends Policy{

//Data Members and Methods

}



Inheritance (2/3)

y The class Policy is known as the super class

parent class

y The class TermInsurancePolicy is knownas

sub class derived class

child class



Policy [Parent Class]

y public class Policy{y private double premium;y private double maturityValue;y //Other Datay public void setPremium(double premium){y this.premium = premium;y }y public double getPremium(){return premium;}y public void setMaturityValue(double maturityValue){y

this.maturityValue = maturityValue;y }y public double getMaturityValue(){return

maturityValue;}y //Other Methodsy }



The protected Access

y The method getBenefit() needs to access the datamembers premium and maturityValue of the class Policy

y A class member that has the protected access specifier canbe accessed by the sub classes also

maturityValue and premium should be declared asprotected

public class Policy{

protected double premium;

protected double maturityValue;

//Other Members

}



Creating the sub class Object

y A TermInsurancePolicy object can invoke all the publicmethods of the class Policy and those that are newly addedin TermInsurancePolicy

y Thus code reusability is achieved

TermInsurancePolicy policy = new TermInsurancePolicy();

policy.setPremium(100);

policy.setMaturityValue(5000);

policy.setTerm(36);

System.out.println(policy.getBenefit());



Inheritance

+setPremium(in premium : double) : void+getPremium() : double

+setMaturityValue(in maturityValue : double) : void

+getMaturityValue(in Amount : double) : void

-premium : double

-maturityValue : double

Policy

+setTerm(in term : int) : void

+getTerm() : int

+getBenifit() : double

-term : int

TermInsurancePolicy

Note: Inheritance is represented by a triangle head arrow in UMLClass diagrams



Multi-Level Inheritance

y A class can be further inherited from a

derived class



Multiple Inheritance

y Concept of a class inheriting from morethan one base class A Hybrid car can inherit from FuelCar andBatteryCar

Multiple inheritance is rarely used because of the complexities it brings in

y Modern OO languages like Java and C#don¶t support Multiple Inheritance



More on Inheritance

y Any number of sub classes can be createdfrom a base class

y Consider a class EndowmentPolicy EndowmentPolicy is a Policy; EndowmentPolicyis extended from Policy

Extra data members and methods are added

public class EndowmentPolicy extends Policy{

//Data Members and Methods

}



Can you answer these questions?

y What is a protected access?

y

What is multilevel inheritance?



Thank You

object orineted programming

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