Difference Between String , StringBuilder and StringBuffer Classes

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 .

String  demo = " hello " ;
// The above object is stored in constant string pool and its value can not be modified.


demo="Bye" ;     //new "Bye" string is created in constant pool and referenced by the demo variable           
 // "hello" string still exists in string constant pool and its value is not overrided but we lost reference to the  "hello"string 

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 .

Due to this it does not allow  two threads to simultaneously access the same method . Each method can be accessed by one thread at a time .

But being thread safe has disadvantages too as the performance of the StringBuffer hits due to thread safe property . Thus  StringBuilder is faster than the StringBuffer when calling the same methods of each class.

StringBuffer value can be changed , it means it can be assigned to the new value . Nowadays its a most common interview question ,the differences between the above classes .
String Buffer can be converted to the string by using
toString() method.

StringBuffer demo1 = new StringBuffer("Hello") ;
// The above object stored in heap and its value can be changed .

demo1=new StringBuffer("Bye");
// Above statement is right as it modifies the value which is allowed in the StringBuffer

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 . 


StringBuilder demo2= new StringBuilder("Hello");
// The above object too is stored in the heap and its value can be modified
demo2=new StringBuilder("Bye");
// Above statement is right as it modifies the value which is allowed in the StringBuilder


----------------------------------------------------------------------------------
                                    String                    StringBuffer         StringBuilder
----------------------------------------------------------------------------------                
Storage Area | Constant String Pool           Heap                       Heap
Modifiable     |  No (immutable)            Yes( mutable )          Yes( mutable )
Thread Safe   |           Yes                                  Yes                              No
 Performance |         Fast                                Very slow                    Fast
-----------------------------------------------------------------------------------


For Example
String s = "Let’s test";
s.concat("if the String object is IMMUTABLE");
System.out.println(s);

O/P:
Let's test.

IF u rewrite the program as below
String s = "Let’s test";
s = s.concat("if the String object is IMMUTABLE");
System.out.println(s);
   
O/P:
Let’s testif the String object is IMMUTABLE

By above example we can say that String is immuatble. It will create new objects every time when ever we assign the values. It will not refer to the old object.


To Overcome this we have StringBuilder  and StringBuffer

    StringBuilder s1 = new StringBuilder("Hello");
    s1.append("Hai");
    System.out.println("StringBuilder : "+s1);
    
    StringBuffer s2 = new StringBuffer("testing");
    s2.append("Hai");
    System.out.println("StringBuffer : "+s2);

O/P:
StringBuilder : HelloHai

StringBuffer : testingHai