String & StringBuffer & StringBuilder

**** String ****

-> String is immutable  ( once created can not be changed )object.
   The object created as a String is stored in the  Constant String Pool  .
   Every immutable object in Java is thread safe ,that implies String is also thread safe .
   String can not be used by two threads simultaneously.
   String  once assigned can not be changed.


**** StringBuffer  ***

-> StringBuffer is mutable means one can change the value of the object .
   The object created through StringBuffer is stored in the heap .
   StringBuffer  has the same methods as the StringBuilder ,
   but each method in StringBuffer is synchronized that is StringBuffer is thread safe .

****** StringBuilder ****

-> StringBuilder  is same as the StringBuffer ,
   that is it stores the object in heap and it can also be modified .
   The main difference between the StringBuffer and StringBuilder is that StringBuilder is also not thread safe.
   StringBuilder is fast as it is not thread safe .