What is an object in Java

Objects are key concepts of java programming, since java is an object oriented programming language. In object oriented programming, program focuses on objects rather than logics. This tutorial explains different details about an object in java.

In real world you can see many examples of object around yourself: a person, your dog, laptop, mobile, bicycle etc. Real world objects have two characteristics, state and behavior. For example a person have state like name, age, height etc and behavior like walk, talk, eat etc while a dog have state like breed, age, color etc and behavior like run, bark, eat etc.

Similarly software object also consist of two characteristics: state and behavior. The state of an object is defined by the instance variables of a class while behavior is defined by the methods of class. So object in java is a collection of data(instance variables) and methods that acts on those data. Programmatically an object is an instance of a class.

So next time when you are creating a class, think of two things about it's objects. "What possible states can this object have?" and "What possible behavior can this object perform?". This way you will get better idea to design your class. You can think of a software object like below which holds it's state and behavior together with itself.

The Syntax given below shows the most basic way of creating an object of a class in java. There are other ways as well but we will not discuss that here.

  ClassName objName = new ClassName();      

Here ClassName is the name of class whose object need to be created and objName is the name of object, given by the programmer. The object name should be unique and should follow the convention given in identifier naming convention. The keyword new is used to create object in java. Using the () with ClassName calls the constructor of that class to create and initialize the object in memory. We will discuss about constructors in later sections. Let's assume we have two classes Person and MyFirstProgram, code given below shows how to create an object of these classes :

  // Creates an object of Person class with name as obj.
  Person obj = new Person(); 
   
  // Creates an object of MyFirstProgram class with name as mfp. 
  MyFirstProgram mfp = new MyFirstProgram();   

Object program in Java

 class Person
   {                           
       // Instance variables, describes state/properties of object of this class
       String name;
       int age;
       int height;         
       // methods, describes behaviors of object of this class
       public void walk() {
            System.out.println("Hi my name is : "+name+", age : "+age+" year,"
            +" height : "+height+" cm. I can Walk");
          }
       public void talk() {
            System.out.println("Hi my name is : "+name+", age : "+age+" year,"
            +" height : "+height+" cm. I can Talk");
          }
       public void eat() {
            System.out.println("Hi my name is : "+name+", age : "+age+" year,"
            +" height : "+height+" cm. I can Eat");
          }                      
   }
      
 class ObjectDemo
   {
     public static void main(String [] args)
       {                   
          // Creating object p1 of Person class
          Person p1 = new Person();
          p1.name = "Rahul";
          p1.age = 20;
          p1.height = 170;
          p1.walk();
          p1.talk();
          p1.eat(); 
          // Creating another object p2 of Person class         
          Person p2 = new Person();
          p2.name = "Rohit";
          p2.age = 30;
          p2.height = 180;
          p2.walk();
          p2.talk();
          p2.eat();
       }
   }

Save above program as ObjectDemo.java
compile as javac ObjectDemo.java
run as java ObjectDemo

Output:

Here p1 and p2 are objects of Person class. Once the code line Person p1 = new Person(); executed, java constructors creates the object p1 in heap memory and assigns the default values of the instance variables for this object. After that code lines p1.name, p1.age and p1.height modifies those default values of the instance variables for this object only. Similarly p2 is also created and assigned the values of instance variables. All the objects of a class are created inside heap memory. You can think of object p1 like below.

At runtime each object that you have created in your program will be allocated space in heap memory. Each object will have it's own copy of instance variables and methods defined inside the class. Changing the value of a instance variable for one object will not change the value of that variable for other object.