This is one of the most popular String Interview questions in Java, which starts with discussion of, What is String, How String in Java is different than String in C and C++, and then shifted towards what is immutable object in Java , what are the benefits of immutable object , why do you use it and which scenarios do you use it. This is some time also asked as "Why String is final in Java" . 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.
