Object Oriented Programming System (OOPS):

Object-Oriented Programming is a technique used to design a program with the help of classes and objects.

• Object
• Class

• Polymorphism
• Inheritance
• Abstraction
• Encapsulation

Object:

It is an entity which has state and behavior. Examples for object are: Fan, table,pen, car etc.

Definition: Object is an instance of a class which has the state which exhibits the well-defined behavior and having unique id.

The characteristics of object:

• State: represents value of an object.(Eg: Car color, Power,name)
• Behavior: represents the behavior of an object such as Car Running State and OFF State etc.
• Identity: Object has unique ID. The JVM identify each object uniquely where the value of the ID is not visible to the external user.

Example: Car is an object, name is Swift and color is Redetcknown as its state. It is used to move from one place to another, so moving is its behavior.

Class:

Class is a collection of object which share common states and the behavior.

A class has the following:

• data member
• method
• constructor

Syntax of a class:

public class JavaProgram
{
  int i=10;   // Data Member
  public void getNumberValue()      //Method
   {
    //Method to get the number value 
   }
}

Constructors:

Constructor is a special type of method which will be implicitly invoked and executed when object is created using “new” keyword. Constructor is mainly used to initialize the data-members.

Syntax: Example

public class JavaProgram
{
  int i=10;   // Data Member
  // I am a default constructor
  public JavaProgram(){
      System.out.println("I will be printed first");
  }
  public void getNumberValue()      //Method
   {
    //Method to get the number value 
   }
}

Characteristics of Constructors:

• Constructor name should be same name as class name
• Constructor does not have any return type, not even void
• Constructor is called at the time of object creation only

Object Creation:

A class is blueprint for objects. Using new keyword will create objects as shown in below example.

public class StringDemo
{
  public void subjectName()
  {
     String name=”Java”;
  }
  public static void main(String[] args)
  {
   StringDemo d1= new StringDemo();  //object creation using new keyword
   /* Note: Here d1 is object reference variable. */
   d1.subjectName();   //calling/executing above method using created object
  }
}

To access instance Variables and Methods using object reference:

Below examples shows how to access the instance variables and methods by using created object.

public class Account {
	String name;
	intact_id;
	Double balance;
	
	void withDraw(double amt)
    {
	   System.out.println("Withdraw amount: "+amt);
    }
    void deposit(double amt)
    {
	   System.out.println("Deposit amount: "+amt);
    }
    public static void main(String[] args) 
	{
		Account a1 = new Account();
		a1.name = "Java";
		a1.act_id = 120;
		a1.balance = 3000.0;
		a1.withDraw(200);
		a1.deposit(100);
	}
} 

The output is shown below:

Java Package:

Packages are the folders which are used to group the classes, interface and files.

Points to remember:

• To declare the package we use package keyword followed by package.
• Package name is reverse of the domain,package name should be in lower case.

Syntax:

1.import <packagename>.*; 
2.import <packagename>.className;

Example: import java.io.*;

• A class can access other classes which are in same package.
• Declaring the package should be first statement in the program.