All the objects that are created must be given initial 
values.There are two approach for this.
1)Using the dot operator to access the instance variables 
and then assign values to them individually---This is a 
tedious approach.
2)Use of getData method to initialize each object 
individually.eg:add.getData(10,15)
To make this simpler Java supports a special type of method 
called a Constructor,that enables an object to initialize 
itself when it is created.Constructor have the same name as 
the class itself and they do not specify a return type,not 
even void,bcoz they return the instance of the class itself.

Constructor is special type of member function that will 
automatically invoke when an object creates. The purpose of 
constructor is for the initialization of instance 
variables. A constructor having the same name as that of 
class name. It may have arguments but doesn't return 
anything. 
  If we want to initialize all objects of the same class 
with same values, use default constructor.Otherwise, we can 
use parameterized constructor if we want to initialize 
objects of the same class with different values. If we 
don't specify any constructor , compiler will use default
constructor with garbage value.

constructor is the one which is used to pass the
initializing parameters during instantiation

Constructor is defined as a type of member function which 
calls itself when an object is created also has the same 
name as of the class.