Objects and Classes

Java is an Object-Oriented Language. As a language that has the Object Oriented feature, Java supports the following fundamental concepts:

• Polymorphism
• Inheritance
• Encapsulation
• Abstraction
• Classes
• Objects
• Instance
• Method
• Message Parsing

In this chapter, we will look into the concepts Classes and Objects.

• Object - Objects have states and behaviors.
   Example: A dog has states - color, name, breed as well as behaviors -wagging, barking, eating. An object is an instance of a class.
• Class - A class can be defined as a template/blue print that describes the behaviors/states that object of its type support.
  
  

  Objects in Java:

  Let us now look deep into what are objects. If we consider the real-world we can find many objects around us, Cars, Dogs, Humans, etc. All these objects have a state and behavior.
  If we consider a dog, then its state is - name, breed, color, and the behavior is - barking, wagging, running
  If you compare the software object with a real world object, they have very similar characteristics.
  Software objects also have a state and behavior. A software object's state is stored in fields and behavior is shown via methods.
  So in software development, methods operate on the internal state of an object and the object-to-object communication is done via methods.
  

  Classes in Java:

  A class is a blue print from which individual objects are created.
  A sample of a class is given below:
  

  public class Dog{
     String breed;
     int age;
     String color;
  
     void barking(){
     }
     
     void hungry(){
     }
     
     void sleeping(){
     }
  }

  A class can contain any of the following variable types.
  
• Local variables: Variables defined inside methods, constructors or blocks are called local variables. The variable will be declared and initialized within the method and the variable will be destroyed when the method has completed.
• Instance variables: Instance variables are variables within a class but outside any method. These variables are instantiated when the class is loaded. Instance variables can be accessed from inside any method, constructor or blocks of that particular class.
• Class variables: Class variables are variables declared with in a class, outside any method, with the static keyword.

A class can have any number of methods to access the value of various kinds of methods. In the above example, barking(), hungry() and sleeping() are methods.
Below mentioned are some of the important topics that need to be discussed when looking into classes of the Java Language.

Constructors:

When discussing about classes, one of the most important sub topic would be constructors. Every class has a constructor. If we do not explicitly write a constructor for a class the Java compiler builds a default constructor for that class.
Each time a new object is created, at least one constructor will be invoked. The main rule of constructors is that they should have the same name as the class. A class can have more than one constructor.
Example of a constructor is given below:

public class Puppy{
   public Puppy(){
   }

   public Puppy(String name){
      // This constructor has one parameter, name.
   }
}

Creating an Object:

As mentioned previously, a class provides the blueprints for objects. So basically an object is created from a class. In Java, the new key word is used to create new objects.
There are three steps when creating an object from a class:

• Declaration: A variable declaration with a variable name with an object type.
• Instantiation: The 'new' key word is used to create the object.
• Initialization: The 'new' keyword is followed by a call to a constructor. This call initializes the new object.

Example of creating an object is given below:

public class Puppy
  {
  public Puppy(String name)
{
  // This constructor has one parameter, name.
 System.out.println("Passed Name is :" + name ); 
 }
  public static void main(String []args)
{ // Following statement would create an object myPuppy Puppy myPuppy = new Puppy( "tommy" ); } 
 }
If we compile and run the above program, then it would produce the following result:
Passed Name is :tommy