Question

As we all know, String is immutable in java. however, one can change it using reflection, by getting the Field and setting access level. (I know it is unadvised, I am not planning to do so, this question is pure theoretical).

my question: assuming I know what I am doing (and modify all fields as needed), will the program run properly? or does the jvm makes some optimizations that rely on String being immutable? will I suffer performance loss? if so, what assumption does it make? what will go wrong in the program

p.s. String is just an example, I am interested actually in a general answer, in addition to the example.

thanks!

Answer 1

You are definitely asking for trouble if you do this. Does that mean you will definitely see bugs right away? No. You might get away with it in a lot of cases, depending on what you’re doing.

Here are a couple of cases where it would bite you:

• You modify a string that happens to have been declared as literal somewhere within the code. For example you have a function and somewhere it is being called like function("Bob"); in this scenario the string "Bob" is changed throughout your app (this will also be true of string constants declared as final).
• You modify a string which is used in substring operations, or which is the result of a substring operation. In Java, taking a substring of a string actually uses the same underlying character array as the source string, which means modifications to the source string will affect substrings (and vice versa).
• You modify a string that happens to be used as a key in a map somewhere. It will no longer compare equal to its original value, so lookups will fail.

I know this question is about Java, but I wrote a blog post a while back illustrating just how insane your program may behave if you mutate a string in .NET. The situations are really quite similar.