String vs StringBuffer vs 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.

String  demo = " hi " ;

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("Hi") ;
// The above object stored in heap and its value can be changed .
demo1.Append(" king! ")
// 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("Hi");
// The above object is stored in the heap and its value can be modified
demo2=new StringBuilder("end");
// Above statement is right as it modifies the value which is allowed in the StringBuilder

import java.lang.*;

public class test {

   public static void main(String[] args) {

   StringBuilder str = new StringBuilder("hi ");
   System.out.println("string = " + str);
 
   // appends the string argument to the StringBuilder
   str.append("king!");
   // print the StringBuilder after appending
   System.out.println("After append = " + str);
 
 
   }
}

output :

string = hi
After append = hi king!

Conclusion :

string vs string biffer vs string builder ?

string string immutable ,

Why string objects are immutable in java?

Because java uses the concept of string literal.Suppose there are 5 reference variables,all referes to one object "sachin".If one reference variable changes the value of the object, it will be affected to all the reference variables. That is why string objects are immutable in java.

StringBuffer class is used to created mutable (modifiable) string. The StringBuffer class in java is same as String class except it is mutable i.e. it can be changed.

StringBuilder class is used to create mutable (modifiable) string. The Java StringBuilder class is same as StringBuffer class except that it is non-synchronized. It is available since JDK 1.5.