//Now to find everywhere this matters and let the caller know. Do you mean callers, who may be threads some of which have decided that the first character of f

//Now to find everywhere this matters and let the caller know. Do you mean callers, who may be threads some of which have decided that the first character of foo should be lowercase while others have been sticking a prefix on foo? How do you coordinate all these separate changes to your foo string? Is there a String manager that coordinates all the threads and decides what the best string update policy is? Maybe I'm not following your argument for mutable strings.