For that to be possible, either you had to cast from mutable to immutable (which you shouldn't have) or the mutable string derives from the immutable string (which it shouldn't have). The latter ...

Strings are the best-known immutable Java objects. Because they never change, the JVM can intern them and cache their hashcodes to minimize the creation of new objects.

Extent-local variables would provide a way to share data both within a thread and with child threads in Java, making it easier to reason about data flow.