why string is immutable in java ?

- String has been widely used as parameter for many java classes e.g. for opening network connection you can pass host-name and port number as string , you can pass database URL as string for opening database connection, you can open any file by passing name of file as argument to File I/O classes.

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.

- 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

String A = "Test"
String B = "Test"

Now String B called "Test".toUpperCase() which change the same object into "TEST" , so A will also be "TEST" which is not desirable.
- Since String is immutable it can safely shared between many threads ,which is very
important for multithreaded programming.

=> Advantages of Immutability

- You can share immutable objects between threads without danger of changes confusing the other thread. You don’t need any locking. Two threads can both work on an immutable object at the same time without any possibility of conflict.
- Once you check the value, you know it has to stay safe. No one can pass you a value, then behind your back swap it to an unsafe one using a background thread. This is particularly important in high security situations where allowing an invalid value to sneak in could compromise system integrity, e.g. a filename. This is probably the main reason that Strings are immutable.
- You can share duplicates by pointing them to a single instance. You need only one copy, e.g. String interning. This saves RAM. With mutable StringBuilders that were temporarily identical, you could not throw one away, and replace it with a reference to the other, since at any moment their values could diverge again.
- You can create substrings without copying. You just create a pointer into an existing base String guaranteed never to change. Immutability is the secret behind Java’s very fast substring implementation.
- Immutable objects are much better suited to be Hashtable keys. If you change the value of an object that is used as a hash table key without removing it and re-adding it you lose the mapping.
- Since String is immutable, inside each String is a char[] exactly the correct length. Unlike a StringBuilder there is no need for padding to allow for growth.


=>To create an immutable class following steps should be followed:

-Create a final class.
-Set the values of properties using constructor only.
-Make the properties of the class final and private
-Do not provide any setters for these properties.
-If the instance fields include references to mutable objects, don't allow those objects to be changed:
-Don't provide methods that modify the mutable objects.
-Don't share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.