Classes & Objects

In Java, everything is associated with classes and objects. In other words, one of the basic concepts of Object-Oriented Programming is classes and objects that revolve around real-life entities.

Class –

• A class is just a group of objects that contains common properties.
• In other words, it is a logical entity.
• A class in Java can contain fields, methods, constructors, blocks, interfaces, etc.
• Syntax – class <class_name>{  //fields and  methods  }  
• Classes are classified into two categories –
  • Built-in Classes
    • Built-in classes are the one that comes bundled within predefined packages in Java and are provided as a part of JDK.
    • Examples –
      • java.lang.String
      • java.lang.System
      • java.lang.Exception
      • java.lang.Object
  • User-Defined Classes
    • A user-defined class is the one that is created by the user.

Object –

• An object is an instance of a class
• In other words, it is a logical as well as a physical entity.
• Object consist of state(data), behavior(functionality) and identity(unique id identified by JVM).
• For instance –
  • a book
    • state – size, color, author
    • behavior – reading
  • a car
    • state – color, type, average, manufacturer
    • behavior – driving
• One can create an object of a class in the following ways –
  • new keyword
  • newInstance() method
  • clone() method

public class Book {
	public Book(String name) {
		System.out.println(name);
	}
	public static void main(String args[]) {
		//object of class Book created using new Operator
		Book book1 = new Book("R.D. Sharma");
		
		//object of class Book created using clone method
		Book book2 = (Book)book1.clone();
		
		//object of class Book created using newInstance method
		Book book3 = (Book) Class.forName("Book").newInstance();
	}
}

Car obj1 = new Car();
Car obj2 = (Car)obj1.clone();