Oop java

Java OOPs Concepts
Simula is considered as the first object-oriented programming
language.
Smalltalk is considered as the first truly object-oriented programming
language.

OOPs (Object Oriented
Programming System)
Object means a real word entity such as pen, chair, table etc. 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:
Object
Class
Inheritance
Polymorphism
Abstraction
Encapsulation

OOPs
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.
Inheritance
When one object acquires all the properties and behaviors of parent
object i.e. known as inheritance. It provides code reusability. It is used
to achieve runtime polymorphism.

OOPs
Polymorphism
When one task is performed by
different ways i.e. known as
polymorphism. For example: to
convense the customer differently, to
draw something e.g. shape or rectangle
etc.
In java, we use method overloading and
method overriding to achieve
polymorphism.
Another example can be to speak
something e.g. cat speaks meaw, dog
barks woof etc.

OOPs
Abstraction
Hiding internal details and showing
functionality is known as abstraction. For
example: phone call, we don't know the
internal processing.
In java, we use abstract class and interface
to achieve abstraction.
Encapsulation
Binding (or wrapping) code and data
together into a single unit is known as
encapsulation. For example: capsule, it is
wrapped with different medicines.
A java class is the example of
encapsulation.

Advantage of OOPs over Procedure-
oriented programming language
1)OOPs makes development and maintenance easier where as
in Procedure-oriented programming language it is not easy to
manage if code grows as project size grows.
2)OOPs provides data hiding whereas in Procedure-oriented
prgramming language a global data can be accessed from
anywhere.
3)OOPs provides ability to simulate real-world event much
more effectively. We can provide the solution of real word
problem if we are using the Object-Oriented Programming
language.

What is difference between object-oriented
programming language and object-based
programming language?
Object based programming language follows all the features of OOPs
except Inheritance.
JavaScript and VBScript are examples of object based programming
languages.

Java Naming conventions
Java naming convention is a rule to follow as you decide what to name
your identifiers such as class, package, variable, constant, method etc.
Name Convention
class name should start with uppercase letter and be a noun e.g. String, Color, Button,
System, Thread etc.
interface name should start with uppercase letter and be an adjective e.g. Runnable,
Remote, ActionListener etc.
method name should start with lowercase letter and be a verb e.g. actionPerformed(),
main(), print(), println() etc.
variable name should start with lowercase letter e.g. firstName, orderNumber etc.
package name should be in lowercase letter e.g. java, lang, sql, util etc.
constants name should be in uppercase letter. e.g. RED, YELLOW, MAX_PRIORITY etc.

Object in Java
An entity that has state and behavior is known as an object e.g. chair, bike,
marker, pen, table, car etc. It can be physical or logical (tengible and
intengible). The example of integible object is banking system.
An object has three characteristics:
state: represents data (value) of an object.
behavior: represents the behavior (functionality) of an object such as
deposit, withdraw etc.
identity: Object identity is typically implemented via a unique ID. The value
of the ID is not visible to the external user. But,it is used internally by the
JVM to identify each object uniquely.
For Example: Pen is an object. Its name is Reynolds, color is white etc.
known as its state. It is used to write, so writing is its behavior.
Object is an instance of a class. Class is a template or blueprint from which
objects are created. So object is the instance(result) of a class.

Class in Java
A class is a group of objects that has common properties. It is a
template or blueprint from which objects are created.
A class in java can contain:
data member
method
constructor
block
class and interface

Method in Java
In java, a method is like function i.e. used to expose behaviour of an
object.
Advantage of Method
Code Reusability
Code Optimization
new keyword
The new keyword is used to allocate memory at runtime.

What are the different
ways to create an object
in Java?
By new keyword
By newInstance() method
By clone() method
By factory method etc.
public static void main(String args[]){
new Calculation().fact(5);//calling method with annonymous object
}
Rectangle r1=new Rectangle(),r2=new Rectangle();//creating two objec
ts

Method Overloading in
Java
If a class have multiple methods by same name but different
parameters, it is known as Method Overloading.
Advantage of method overloading?
Method overloading increases the readability of the program.
Different ways to overload the method
By changing number of arguments
By changing the data type

1)Example of Method
Overloading by changing the
no. of arguments
class Calculation{
void sum(int a,int b){System.out.println(a+b);}
void sum(int a,int b,int c){System.out.println(a+b+c);}
Calculation obj=new Calculation();
obj.sum(10,10,10);
obj.sum(20,20);
}
}

2)Example of Method Overloading
by changing data type of argument
class Calculation2{
void sum(int a,int b){System.out.println(a+b);}
void sum(double a,double b){System.out.println(a+b);}
Calculation2 obj=new Calculation2();
obj.sum(10.5,10.5);
obj.sum(20,20);
}
}

class Calculation3{
int sum(int a,int b){System.out.println(a+b);}
double sum(int a,int b){System.out.println(a+b);}
Calculation3 obj=new Calculation3();
int result=obj.sum(20,20);
}
}

Can we overload main()
method?
class Overloading1{
public static void main(int a){
System.out.println(a);
}
System.out.println("main() method invoked");
main(10);
}
}

Method Overloading and
TypePromotion
One type is promoted to another implicitly if no matching datatype is
found.

Example of Method
Overloading with
TypePromotion
class OverloadingCalculation1{
void sum(int a,long b){System.out.println(a+b);}
void sum(int a,int b,int c){System.out.println(a+b+c);}
OverloadingCalculation1 obj=new OverloadingCalculation1();
obj.sum(20,20);//now second int literal will be promoted to long
obj.sum(20,20,20);
}
}

Example of Method Overloading with
TypePromotion if matching found
void sum(int a,int b){System.out.println("int arg method invoked");}
void sum(long a,long b)
{System.out.println("long arg method invoked");}
obj.sum(20,20);
}
}

Example of Method Overloading with
TypePromotion in case ambiguity
void sum(int a,long b){System.out.println("a method invoked");}
void sum(long a,int b){System.out.println("b method invoked");}
obj.sum(20,20);//now ambiguity
}
}

Constructor in Java
Constructor in java is a special type of method that is used to initialize
the object.
Rules for creating java constructor
Constructor name must be same as its class name
Constructor must have no explicit return type

Types of java constructors
Default constructor (no-arg constructor)
Parameterized constructor

Example of default
constructor
class Bike1{
Bike1(){System.out.println("Bike is created");}
Bike1 b=new Bike1();
}
}
If there is no constructor in a class, compiler automatically creates a
default constructor.

What is the purpose of default constructor?
Default constructor provides the default values to the object like 0, null
etc. depending on the type.

Java parameterized
constructor
Why use parameterized constructor?
Parameterized constructor is used to provide different values to the
distinct objects

class Student4{
int id;
String name;
Student4(int i,String n){
id = i;
name = n;
}
void display(){System.out.println(id+" "+name);}
Student4 s1 = new Student4(111,"Karan");
Student4 s2 = new Student4(222,"Aryan");
s1.display();
s2.display();
}
}

Constructor Overloading in Java
class Student5{
int id;
String name;
int age;
id = i;
name = n; }
Student5(int i,String n,int a){
id = i;
name = n;
age=a; }
void display(){System.out.println(id+" "+name+" "+age);}
Student5 s2 = new Student5(222,"Aryan",25);
s1.display();
s2.display();
}

Difference between
constructor and method in
java
Java Constructor Java Method
Constructor is used to initialize
the state of an object.
Method is used to expose
behaviour of an object.
Constructor must not have return
type.
Method must have return type.
Constructor is invoked implicitly. Method is invoked explicitly.
The java compiler provides a
default constructor if you don't
have any constructor.
Method is not provided by
compiler in any case.
Constructor name must be same
as the class name.
Method name may or may not be
same as class name.

Java Copy Constructor
There is no copy constructor in java. But, we can copy the values of one
object to another like copy constructor in C++.
There are many ways to copy the values of one object into another in
java. They are:
By constructor
By assigning the values of one object into another
By clone() method of Object class

class Student6{
int id;
String name;
id = i;
name = n;
}
Student6(Student s){
id = s.id;
name =s.name;
}
void display(){System.out.println(id+" "+name);}
Student6 s2 = new Student6(s1);
s1.display();
s2.display();
}
}

Copying values without
constructor
Student7 s2 = new Student7();
s2.id=s1.id;
s2.name=s1.name;
s1.display();
s2.display();
}
}

Does constructor return
any value?
yes, that is current class instance (You cannot use return type yet it
returns a value).
Can constructor perform other tasks instead of initialization?
Yes, like object creation, starting a thread, calling method etc. You can
perform any operation in the constructor as you perform in the
method.

Java static keyword
The static keyword in java is used for memory management mainly. We
can apply java static keyword with variables, methods, blocks and
nested class.
The static can be:
variable (also known as class variable)
method (also known as class method)
block
nested class

Java static variable
If you declare any variable as static, it is known static variable.
The static variable can be used to refer the common property of all
objects (that is not unique for each object) e.g. company name of
employees, college name of students etc.
The static variable gets memory only once in class area at the time of
class loading.
Advantage of static variable
It makes your program memory efficient (i.e it saves memory).

class Student8{
int rollno;
String name;
static String college ="ITS";
Student8(int r,String n){
rollno = r;
name = n;
}
void display (){System.out.println(rollno+" "+name+" "+college);}
Student8 s2 = new Student8(222,"Aryan");
s1.display();
s2.display();
}
}

class Counter{
int count=0;//will get memory when instance is created
Counter(){
count++;
System.out.println(count);
}
Counter c1=new Counter();
}
}

Java static method
A static method belongs to the class rather than object of a class.
A static method can be invoked without the need for creating an
instance of a class.
static method can access static data member and can change the value
of it.

class Student9{
int rollno;
String name;
static String college = "ITS";
static void change(){
college = "BBDIT";
}
Student9(int r, String n){
rollno = r;
name = n;
}
void display (){System.out.println(rollno+" "+name+" "+college);}
Student9.change();
Student9 s1 = new Student9 (111,"Karan");
Student9 s2 = new Student9 (222,"Aryan");
Student9 s3 = new Student9 (333,"Sonoo");
s1.display();
s2.display();
s3.display();
}

Oop java

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Oop java