Object-Oriented Programming is a
methodology or paradigm to design a program using classes and objects. It
simplifies the software development and maintenance by providing some concepts:
v Object
v Class
v Inheritance
v Polymorphism
v Abstraction
v Encapsulation
Object:
Any entity that has state and behavior is known
as an object. For example: chair, pen, table, keyboard, bike etc. It can be
physical and logical.
Class:
Collection of objects is called class. It
is a logical entity.
Example:
public class
ClassAndObjectDemo {
public static void
main(String[] args) {
// Creation of
an Object. we can creates n number of Objects for a single class
Sample s = new
Sample();
}
}
class
Sample{// class
int i;
float j;
Sample(){
System.out.println("Sample
Class Constructor");
}
void
m1(){
System.out.println("sample
class m1");
}
}
Important Point:
1. For
creating an object we are using a 'new' operator
2. Each object will have
all the features of a class
Abstraction:
If we want to hide the implementation to the
outside world we are using abstraction. With this technique we are providing
security to our implementation.
In
JAVA we are achieving this abstraction in two ways
1. By creating abstract
class
2. By creating Interface
Abstract
class:
An abstract
class is a class which will have abstract methods and concrete methods.
Abstract method: This is a
method without implementation
Concrete method: This is a method
which will have implementation
Ex: Abstract class and its allowed
elements in an abstract class
public abstract class
AbstractClassDemo {
// we can create instance
variables
int i;
int j;
// we can create constants
final int k=10;
// we can create a constructor
for abstract class
public
AbstractClassDemo() {
System.out.println("AbstractClassDemo
Constructor");
}
// abstract method
abstract void
m1();
//concrete method
private void m2(){
}
// we can write a main method
public static void
main(String[] a){
System.out.println("Raju");
}
Important Points:
1. We
are not allowed to create an object for abstract class but we can creates
reference to abstract class
Ex: AbstractClassDemo
aCD = new AbstractClassDemo(); // Wrong
AbstractClassDemo aCD; //
it allowed. Here we are just creating reference
2. We
can’t create object for the abstract class.
3. We
can create reference variable for abstract class.
4. If
a class contains abstract method then we should declare that class as an
abstract class.
5. We
can extend abstract class to another class.
6. If
extended class also having abstract class then that class also should declared
as abstract class.
7. Abstract
class may contain zero no.of abstract methods. This is for avoiding object
creation.
Interfaces:
An
interface will have only abstract methods. So we can call this interface is a
fully abstract class
Ex:
Interface and allowed elements
public interface
InterfaceDemo {
//we can declare constants
// if we are not declare "public
final static" for constants then compiler by default includes
public final static int CONSTANT_ONE = 1;
// if we are not declare
"public abstract" for methods then compiler by default includes
public abstract void
methodOne();
public abstract int
methodTwo();
}
Important Points:
1. We
are not allowed to create an object for abstract class but we can creates
reference to abstract class
Ex: InterfaceDemo
iD = new InterfaceDemo ();
InterfaceDemo iD;
it allowed. Here we are just creating
reference
it allowed. Here we are just creating
reference
2. We
never highlight our implementation to the outside world so we can achieve
security for our implementation.
3. Interface
contains 100% abstract methods.
4. We
can declare constants.
5. By
default these constants ‘public static final’.
6. By
default methods have ‘public abstract’
Inheritance:
If
we want to acquire all the features from other component into our component we
will go for inheritance. In JAVA we can achieve this inheritance in two ways
1. By extending a class :
For this we are using “extends”
keyword
Ex:
public class
ExtendsDemo extends Sample{
public static void
main(String[] args) {
// TODO
Auto-generated method stub
Sample s = new
Sample();
s.method1();
}
}
class
Sample{
int
method1(){
System.out.println("I
am Sample method1");
return 'a';
}
}
Important Points:
1. We
can extend only one class to another class.
Ex:
class
ExtendsDemo extends Sample
class
ExtendsDemo extends Sample,Sample1
class ExtendsDemo extends Sample extends Sample1
2. We
can’t achieve multiple inheritance in JAVA using classes because of ambiguity
problem
2.
By implementing an interface :
For this
we are using “implements” keyword
Ex:
public interface
InterfaceDemo {
//we can declare constants
// if we are not declare
"public final static" for constants then compiler by default includes
public final static int CONSTANT_ONE = 1;
// if we are not declare
"public abstract" for methods then compiler by default includes
public abstract void
methodOne();
public abstract void
methodTwo();
}
class
ImplementsDemo implements InterfaceDemo{
public void
methodOne(){
System.out.println("methodOne
Implementation");
}
// if we are not interested to
provide implementation then we should provide an empty implementation at least
public void
methodTwo(){
}
}
Important Points:
1. We
can achieve multiple inheritance by using interfaces
Ex:
class
ImplementsDemo implements InterfaceDemo1,InterfaceDemo2
class ImplementsDemo implements InterfaceDemo1 implements InterfaceDemo2
class ImplementsDemo extends Sample implements InterfaceDemo2
class ImplementsDemo extends Sample implements InterfaceDemo2
Polymorphism:
Polymorphism means one thing is in many ways. In
JAVA we can achieve in two ways
1.
By using method overloading
Ex:
public class
MethodOverLoadingDemo {
// this is called overloading
method
public void
methodOne(int i){
System.out.println("I
am MethodOverLoadingDemo methodOne()-->"+i);
}
}
class MethodOverloadingImpl
extends MethodOverLoadingDemo{
public static void
main(String[] args) {
// calling
child class method with child class reference with child class object
MethodOverloadingImpl
methodOverloadingImpl= new
MethodOverloadingImpl();
methodOverloadingImpl.methodOne();
//calling
parent class method with child class reference with child class object
methodOverloadingImpl.methodOne(0);
// calling
child class method with parent class reference with child class object
// In method Overloading
method calls will be decides based on reference not on based on created object
MethodOverLoadingDemo
methodOverLoadingDemo = new
MethodOverloadingImpl();
methodOverLoadingDemo.methodOne(0);
//methodOverLoadingDemo.methodOne();
-- this is not possible
}
// this is overloaded method
public void
methodOne() {
System.out.println("I
am MethodOverloadingImpl methodOne");
}
}
Output:
I am
MethodOverloadingImpl methodOne
I am
MethodOverLoadingDemo methodOne()-->0
I am MethodOverLoadingDemo
methodOne()-->0
Important Points:
1. This
is called static polymorphism.
2. All
the method calls will decides based on references not based on objects
3. Two
methods are said to be overloaded if they have the same method names but
different parameter list (At least order).
4. We
never consider return type, access modifier and throws clause in overloading.
5. In
method overloading, which method need to call decided by the compiler at
compile time. So overloading is called “static polymorphism” OR “early
binding”
2.
Method Overriding
Ex:
public class
MethodOverridingDemo {
//This is overriding method
public int
methodOne(){
System.out.println("I
am MethodOverridingDemo methodone()");
return 10;
}
}
class
MethodOverridingImpl extends
MethodOverridingDemo{
//This is overridden method
// In method Overriding method
signature should be same
public int
methodOne() {
System.out.println("I
am MethodOverridingImpl methodOne()");
// return
value can be different data type which can be internally promoted to declared
data type
return 'a';
}
public static void
main(String[] args) {
// calling
child class method with child class reference with child class object
MethodOverridingImpl methOverridingImpl
= new MethodOverridingImpl();
methOverridingImpl.methodOne();
// calling
child class method with Parent class reference with child class object
// In method
Overriding method calls will be decides based on created object not based on
reference
MethodOverridingDemo
methodOverridingDemo = new
MethodOverridingImpl();
methodOverridingDemo.methodOne();
}
}
Output:
I am
MethodOverridingImpl methodOne()
I am MethodOverridingImpl methodOne()
Important Points:
1. In
overriding the method names and arguments (including order must be same).
2. In
case of overriding signatures of the methods must be same.
3. Access
modifier incremental is allowed in method overriding.
4. An
abstract method can be overridden as non-abstract method
5. While overriding we can’t increase the no.of
checked exceptions & we can‘t directly use parent exception.
6. There
is no rule for unchecked exceptions
Encapsulation:
Encapsulation
means bundling the data into a one unit. In JAVA we have two types of
encapsulations
1. Partial encapsulation :
class is the best example for partial encapsulation because a call can contains
all kinds of elements with all kinds of access modifiers
2. Fully encapsulation: JAVA
bean is the best example for full encapsulation because all the members are
private and getters and setters are public access modifiers
Ex: Bean example
public class
JavaBeanDemo {
private int sno;
private String sname;
public int getSno()
{
return sno;
}
public void setSno(int sno) {
this.sno = sno;
}
public String
getSname() {
return sname;
}
public void
setSname(String sname) {
this.sname = sname;
}
}
Important Points:
1. All
the members should be private
2. Setters
don’t have return type
3. Setters
have private access
4. Getters
have public access modifier
5. Getters
have return type of corresponding data member data type declared
6. Getters
have return type whereas setters don’t
7. Setters
have parameter whereas getter doesn’t
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