effect of changing String using reflection

After compilation some strings may refer to the one instance, so, you will edit more than you want and never know what else are you editing.

public static void main(String args[]) throws Exception {
    String s1 = "Hello"; // I want to edit it
    String s2 = "Hello"; // It may be anywhere and must not be edited
    Field f = String.class.getDeclaredField("value");
    f.setAccessible(true);
    f.set(s1, "Doesn't say hello".toCharArray());
    System.out.println(s2);
}

Output:

Doesn't say hello

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.