CS_IS_F213_L2_23May

BITS Pilani Hyderabad Campus

BITS Pilani Ms. Prafulla Kalapatapu

Computer Science & Information Systems Group

BITS-Pilani Hyderabad Campus

[email protected]

BITS Pilani Hyderabad Campus

Object Oriented Programming Summer term Semester 2015

CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani, Hyderabad Campus

Encapsulation

Todays Agenda


ENCAPSULATION


What is an Encapsulation?

Combining data and code at one place as a module.

How can we manage the data?

Using variables/fields/attributes

How can we manage the code?

Using methods/ functions

How can we implement Encapsulation in Java?

Using classes and objects

Encapsulation


What is a class?

- Class is an user defined type, which contains variables and methods.

- It is a blue print/template for creating objects.

- It doesnt exist physically (i.e memory wont be allocated).

What is an Object?

Object is an instance of class

Syntax

class classname

{

}

Cont..

User defined name Keyword

Variables/fields/properties

+

Methods/operations/behaviour

Class body


For Class Name :

- Class name starts with a capital letter

- If class name contains multiple words each word should start with capital letter.

For Variables :

- Variable names should be in small letters.

For Constants:

- Constant variable names should be in capital letters

For Methods:

- Method name starts with a small letter

- From second word onwards,each words first letter will be in capital letter

Name Conventions

Ex: Employee, Student

Ex: HelloWorld

Ex: final int PI=3.14;

Ex: taxCalculation();

Ex: int total;


Purpose is to store the data

Variables

1. Instance Variables:

If we declare any variable in class, then we can call those variables

as instance variables.

- When memory allocates for instance variable?

When we create an instance/object, at that time, memory will

allocate for instance variable.

- Can we use multiple instances for a class?

Yes (any no.of)

Variables

1. Instance variables/object level/attributes

2. Static variables/class level.


2. Static Variables:

If we declare any variable with a keyword static, in a class, we can call those variables as static variables.

- When memory allocates for static variable?

When the class is loaded into the memory, at that time, memory

will be allocated to static variable.

Similarities between instance and static variables

Both are used to store data

Variables [1]


Difference between instance and static variables.

Instance Variables Static Variables

No static keyword Declaration with keyword static

Memory allocated when object

is created

Memory is allocated when class

is loaded into memory

For each object, separate copy

of instance variable will be

created

For all objects, only one copy

of static variable created

Memory is created in Heap Memory is created in Method

area

Variables [2]


What is a method?

If we write a function in a class,we can call it as method.

What is the purpose of method?

Purpose is to perform some task.

Syntax:

returntype methodname()

{

//method body

}

Method


Method Header or Method prototype: It contains

Method Signature: It contains

Method [1]

Return type

Method name

Method Parameters

Method name

Method Parameters


Methods

1. Instance Method:

Defining /writing any method in a class, we can call it as instance

method.

- How we can call instance method?

Using object name.

Syntax: objectname.method();

v1.calc();

- To call instance method, we should create an object.

Method [2]

1. Instance Methods.

2. Static Methods.


2. Static Method:

Defining/writing a method with the keyword static, we can call it as static method.

- How we can call static method?

Using classname

Syntax: classname.methodname();

Ex: Sample.m1();

- Limitation of static method:

static void m1()

{

// cant access instance variables/methods

}

- what static method can access?

static variables/methods

local variables

Method [3]


Reason for not accessing instance variables in static methods:

First when class is loaded, for all static variables/methods/blocks, memory will be allocated in the method area.

Later, objects/instance will be created in the heap area.

So, later part [objects/heap area] cant be accessed in prior part [method area].

But prior part [method area] can be accessed in later part [objects/heap area].

Method [4]


class ABC

{

public static void main(String []a)

{

System.out.println(hello);

}

}

Suppose we have saved above program with ABC.java

Example:

User defined classname

Keyword

Class

body

Class name

Pre-defined classname

Method Prototype

Variable in

System class

Method


Compilation & Execution process of JAVA

ABC.java Java

Compiler Byte code ABC.class

JVM Machine

code

Machine

Independent

Machine

Dependent

What is the responsibilitty of JVM?

It takes .class file and converts each byte code instruction into the

machine code with respective to individual OS


class Employee

{

int eid;

String ename;

double esal;

Static int empcount;

void displayEmpDetails()

{

System.out.println(eid+ +ename);

}

static void dispEmpCount()

{

System.out.println(empcount); //eid,ename,esal cant be accessed

}

}

Example for class:

User defined classname

Keyword

Class

body

Instance method

Static method

Instance variables

Static variable


What is an Object?

- Object is an instance of a class

- Object exist physically.[heap memory]

What is Instantiation?

Process of creation of an object.

How can we create an Object?

Using new keyword.

Syntax to create an Object

classname refname=new classname();

Object

To hold an object Object creation (4 Step Process)


Employee e1 = new Employee();

Employee is a class name.

e1 is a reference variable of type Employee, to hold employee object (RHS).

In RHS, object will create (4 step process).

Alternate way to represent the same

Employee e1;

e1 = new Employee();

Example


Ex:

class Sample

{

int a=10;

String s;

boolean b;

public static void main(String []args)

{

}

}

4 Step Object Creation Process

Sample s1=new Sample();

Object Creation Process Step 3

Step 2

Step 4

Step 1


In RHS, It will create memory for all instance variables and initializes with

default values.

Representation of object in Memory :

Step 1

0

false null

a

s b

Data

Type

Default

Value

int 0

float 0.0

double 0.0

string Null

char \u0000

boolean false

Any class

type

variable

null


Assign instance variable with given values

Representation in memory :

Step 2

10

false null

a

s b


3. Constructor will be called.

- Constructor is a special method.

4. Last step returns memory reference/address.

Step 3 & 4


What is a constructor?

It is a special method.

What is the purpose of the constructor?

To initialize objects state. (Assigning values to instance variables).

Rules to write a constructor.

- classname == methodname

- It should not have return type

Constructor


Constructor Vs Method

Constructor Method

It is a special method i.e it

calls implicitly when object is

going to be created as a 3rd

step.

It is a method i.e it calls

explicitly

classname == methodname It can be any name

It should not have return type It should have return type

Purpose is to intialize object

state

Purpose is to perform some

task


If we dont write a constructor, java compiler will write default constructor

Ex:

class Abc

{

}

In the above class Abc, we didnt write any constructor, when you compile this class, your java compiler will write default constructor as shown below.

Ex:

class Abc

{

}

Case (i)

Abc()

{

super(); }

Written by the

java compiler

It calls the super

class constructor

Note: Any class it is implicitly

derived from an Object class


If we write a constructor in the class

Ex:

class Abc

{

int a;

Abc()

{

a=20;

}

}

After compiling above class, your java compiler will add one statement as first line in the constructor

What is the responsibility of super();

super() calls the super classs constructor.

Case (ii)

super(); Written by java compiler after compiling

super();

NOTE: Default constructor is

written by java compiler, only

when there is no constructor in

the class

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CS_IS_F213_L2_23May