Java Tutorials

Immutable String

In Java, String is a class and objects of this class is used to store strings.These String objects are made immutable in Java.Immutable means unchangeable, which cannot be changed.Once an object of String class is created, its content cannot be changed or altered.Whenever the program tries to make any modification to any String class object, the value residing in it is not changed but rather a new String object is created with the desired value and the old reference variable is made to point at the newly created object.Let's see this example

class StringTutorial
 {  
  public static void main(String args[])
    {  
     String s="Robert";  
     s.concat("James");//Using concat function to Add "James" at the end of "Robert"
     System.out.println(s);//will print Robert because strings are immutable objects and original state cannot be changed!
    }  
  } 

Output:

Robert 

In the above example, we are trying to add the string "James" at the end of "Robert". After we have used concat function to add the two Strings the output is surprisingly still the same.Why did it happen?

It happened because String objects are immutable and cannot be changed and 's' represents a String object holding the value "Robert".Whatever changes we make to the content of 's' will not be reflected in it instead a new object will be created in memory containing the changes we wanted to do and the reference variable 's' will be made to point to that new object.

Here 's' was pointing to "Robert" and when we used the concat function to add "James" at the end of "Robert" the original object was not affected but a new object was created with the changes we wanted.
Did you notice that 's' is still pointing to the old object? and this is the reason why the output was still "Robert" not "RobertJames" because we forgot to make 's' point to the newly created object.If we want the concatenated output then we should do this

class StringTutorial
  {  
   public static void main(String args[])
    {  
       String s="Robert";  
       s=s.concat("James");//Using concat function to Add "James" at the end of "Robert"
       System.out.println(s);//will print RobertJames
       }  
    }  

Why Strings are Immutable in Java?
The exact reasons for making strings immutable in java can be stated only by the the creators of String class.There can be many possible answers to this question but let's see some reasonable arguments :
1)Required by String constant pool:
String pool is a special storage area in Java heap memory. Whenever a string is created and that newly created string already exists in the String constant pool, the reference of the existing string will be returned and  assigned to the reference variable associated with the new string, instead of creating a new object and returning its reference.
The following code will create only one string object in the heap

String string1 = "Hello";
String string2 = "Hello";

If string is not immutable, changing any one string will lead to the wrong value for the other references.

2)Security:
3)To make Strings thread-safe:
4)Caching hashcode:
5)Class Loading Mechanism: