Class & Objects

• Class
• Classes and Objects are basic concepts of Object-Oriented Programming.
• A class is a user-defined data type from which objects are created.
• A class can be defined as a template/blueprint that describes the behavior/state that the objects.
• Syntax:

modifier class class_name
{
member variable (properties);
member methods;
}

• It represents the set of properties or methods.
• Modifiers : A class can be public or private or default access.
• The private keyword makes instance variables and methods private which can be accessed only from inside the same class.
• The public keyword makes instance variables and methods public which can be accessed from outside of the class.
• class: It is a keyword.
• class_name: The name should begin with an initial letter.
• Body (block of code): The class body surrounded by braces { }.
• Class has mainly two types of members:
• 1. member variable (states)
• 2. member methods (behaviors)
• Example:

public class Dog {
   String breed;
   int age;
   String color;

   void barking() {
   }

   void hungry() {
   }

   void sleeping() {
   }
}

Note: When class is defined, no memory or storage is allocated.

• Object
• It is a basic unit of OOP.
• A Java program creates many objects from class.
• Objects have states and behaviors.
• An object has:
• States : 
• It is represented by attributes of an object. 
• It also reflects the properties of an object.
• Behavior: 
• It is represented by methods of an object.
• It also reflects the response of an object with other objects.
• Identity:  
• It gives a unique name to an object.
• One object to interact with other objects.
• Example: 
• A dog has states - color, name, breed as well as behaviors – wagging the tail, barking, eating.

Creating & Declaring Objects:

• A single class may have any number of instances
• All the instances share the attributes and the behavior of the class.
• Creating an Object
• A class provides the blueprints for objects. 
• So basically, an object is created from a class. 
• In Java, the new keyword is used to create new objects.
• There are three steps when creating an object from a class −
• 1. Declaration − A variable declaration with a variable name with an object type.
• 2. Instantiation − The 'new' keyword is used to create the object.
• 3. Initialization − The 'new' keyword is followed by a call to a constructor. This call initializes the new object.
• Example:

Dog myDog = new Dog();
Dog yourDog = new Dog();

Types of variables:

• Local variables − Variables defined inside methods, constructors or blocks are called local variables. 
• Instance variables − Instance variables are variables within a class but outside any method. 
• Class variables − Class variables are variables declared within a class, outside any method, with the static keyword.

How to access members?

• You can access members (call methods and access instance variables) by using . operator. 
• Syntax:

object_name.members_variable;
object_name.members_method;

• For example:

myDog.breed;
yourDog.color;

myDog.sleeping();
yourDog.barking();