Why java string is immutable

 Why java string is immutable?
This is one of the most popular interview question on String in Java which starts with discussion of What is immutable object , what are the benefits of immutable object , why do you use it and which scenarios do you use it.

It can also come once interviewee answers some preliminarily strings questions e.g. What is String pool , What is the difference between String and StringBuffer , What is the difference between StringBuffer and StringBuilder etc.

Though there could be many possible answer for this question and only designer of String class can answer this , I think below two does make sense

1)Imagine StringPool 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.

2)String has been widely used as parameter for many java classes e.g. for opening network connection you can pass hostname and port number as stirng , 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.

3)Since String is immutable it can safely shared between many threads ,which is very
important for multithreaded programming.

I believe there could be some more very convincing reasons also , Please post those reasons as comments and I will include those on this post.