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BITS Pilani Hyderabad Campus
BITS Pilani Ms. Prafulla Kalapatapu
Computer Science & Information Systems Group
BITS-Pilani Hyderabad Campus
[email protected]
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BITS Pilani Hyderabad Campus
Object Oriented Programming Summer term Semester 2015
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
Encapsulation
Abstraction
Todays Agenda
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
ENCAPSULATION
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
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)
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
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
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
Ex:
class Sample
{
int a=10;
String s;
boolean b;
public static void main(String []args)
{
}
}
4 Step Object Creation Process
Sample s=new Sample();
Object Creation Process Step 3
Step 2
Step 4
Step 1
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
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
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
Assign instance variable with given values
Representation in memory :
Step 2
10
false null
a
s b
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
3. Constructor will be called.
- Constructor is a special method.
4. Last step returns memory reference/address.
Step 3 & 4
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
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
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
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
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
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
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
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 F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
ABSTRACTION
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
What is an Abstraction?
Hiding essential details and unhiding the non-essential
details.
How can we implement Abstraction
Using Access specifiers.
What is an Access specifier?
It specifies the accessibility.
How many Access specifiers are there?
4 Access specifiers
1. private
2. public
3. protected
4. default
Abstraction
Three are
keywords
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
Private members can't access outside the class.
1. private
class Abc { private int x,y; }
class Bbc { }
Access within
the class Cant access
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
2. public
Public members can access anywhere in the Java program.
class Abc { public int x,y; }
class Bbc { }
Access to all
the class Can access
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
Protected members can be accessed when there is relation between
classes (when classes in different packages).
Protected members can be accessed between the classes, even if
there is no relation (classes with in the same package).
3. protected
class Abc { protected int a; }
class Xyz { }
p1 p2
class Abc{ protected int a; } class Xyz { -----}
p1
If and only if Xyz
extends Abc
Can be accessed
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
Default members can be accessed with in the same package
only.
4. default
p1
Can access
default members Abc
Xyz
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CS F213 OOP Summer Term 2015 Prafulla Kalapatapu BITS Pilani,
Hyderabad Campus
1. Private within the class
2. Public anywhere in Java environment [even if it is non-child
class]
3. Protected same package+ other package[only if there is a
relation between classes]
4. Default within the same package/folder.
Very Imp : Encapsulation and Abstraction goes hand in hand
Summary- Access specifiers
