• In java, string is basically an object that represents sequence of char values. An array of characters works same as java string.
• Generally, string is a sequence of characters. But in java, string is an object that represents a sequence of characters. The java.lang.String class is used to create string object.
• Declaration :

String s1="Welcome";  
String s2="Welcome";//will not create new instance
String s=new String("Welcome");

• In java, string objects are immutable. Immutable simply means unmodifiable or unchangeable.Once string object is created its data or state can't be changed but a new string object is created.
• There are three ways to compare string in java:
  1.By equals() method
  2.By = = operator
  3.By compareTo() method

eg:

The String equals() method compares the original content of the string. It compares values of string for equality. 

class Teststringcomparison1{  
 public static void main(String args[]){  
   String s1="Sachin";  
   String s2="Sachin";  
   String s3=new String("Sachin");  
   String s4="Saurav";  
   System.out.println(s1.equals(s2));//true  
   System.out.println(s1.equals(s3));//true  
   System.out.println(s1.equals(s4));//false  
 }  
}  

The = = operator compares references not values. 

class Teststringcomparison3{  
 public static void main(String args[]){  
   String s1="Sachin";  
   String s2="Sachin";  
   String s3=new String("Sachin");  
   System.out.println(s1==s2);//true (because both refer to same instance)  
   System.out.println(s1==s3);//false(because s3 refers to instance created in nonpool)  
 }  
}  

The String compareTo() method compares values lexicographically and returns an integer value that describes if
first string is less than, equal to or greater than second string.

String Buffer class

• Java StringBuffer class is used to create mutable (modifiable) string. The StringBuffer class in java is same as String class except it is mutable i.e. it can be changed.
• Java StringBuffer class is thread-safe i.e. multiple threads cannot access it simultaneously. So it is safe and will result in an order.

String Builder class

• Java StringBuilder class is used to create mutable (modifiable) string. The Java StringBuilder class is same as StringBuffer class except that it is non-synchronized.
• StringBuffer is less efficient than StringBuilder.