Why String is immutable in Java?

A classic example of an immutable object is an instance of the Java String class.

    String s = "ABC";
    s.toLowerCase();

The method toLowerCase() will not change the data "ABC" that s contains. Instead, a new String object is instantiated and given the data "abc" during its construction. A reference to this String object is returned by the toLowerCase() method. To make the String s contain the data "abc", a different approach is needed.

     s = s.toLowerCase();

Now the String s references a new String object that contains "abc". There is nothing in the syntax of the declaration of the class String that enforces it as immutable; rather:

   ----> none of the String class's methods ever affect the data that a String object contains, thus making it INMUTABLE.

1) Imagine String pool facility without making string immutable , its not possible at all because in case of string pool one string object/literal e.g. "Test" has referenced by many reference variables , so if any one of them change the value others will be automatically gets affected i.e. lets say


Read more: http://javarevisited.blogspot.com/2010/10/why-string-is-immutable-in-java.html#ixzz3MUEvTdDI

2. In case, if String is not immutable, this would lead serious security threat , I mean some one can access to any file for which he has authorization, and then can change the file name either deliberately or accidentally and gain access of those file.

Read more: http://javarevisited.blogspot.com/2010/10/why-string-is-immutable-in-java.html#ixzz3MUFPxTaH

3)Since String is immutable it can safely shared between many threads ,which is very important for multithreaded programming and to avoid any synchronization issues in Java,

Read more: http://javarevisited.blogspot.com/2010/10/why-string-is-immutable-in-java.html#ixzz3MUFkRtwj

4) Another reason of Why String is immutable in Java is to allow String to cache its hashcode , being immutable String in Java caches its hashcode, and do not calculate every time we call hashcode method of String, which makes it very fast as hashmap key to be used in hashmap in Java.

Read more: http://javarevisited.blogspot.com/2010/10/why-string-is-immutable-in-java.html#ixzz3MUFsc4AI

5) "The absolutely most important reason that String is immutable is that it is used by the class loading mechanism, and thus have profound and fundamental security aspects. Had String been mutable, a request to load "java.io.Writer" could have been changed to load "mil.vogoon.DiskErasingWriter""

Read more: http://javarevisited.blogspot.com/2010/10/why-string-is-immutable-in-java.html#ixzz3MUKGdd2h

http://javarevisited.blogspot.sg/2012/07/when-class-loading-initialization-java-example.html


Also to be immutable you have to be final, so that your subclass doesn't break immutability

Read more: http://javarevisited.blogspot.com/2010/10/why-string-is-immutable-in-java.html#ixzz3MULmh42T