It is very useful to have strings implemented as final or immutable objects. Below are some advantages of String Immutability in Java
- Immutable objects are thread-safe. Two threads can both work on an immutable object at the same time without any possibility of conflict.
- Security: the system can pass on sensitive bits of read-only information without worrying that it will be altered
- You can share duplicates by pointing them to a single instance.
- You can create substrings without copying. You just create a pointer into an existing base String guaranteed never to change. Immutability is the secret that makes Java substring implementation very fast.
- Immutable objects are good fit for becoming Hashtable keys. If you change the value of any object that is used as a hash table key without removing it and re-adding it you will lose the object mapping.
- Since String is immutable, inside each String is a char exactly the correct length. Unlike a StringBuilder there is no need for padding to allow for growth.
- If String were not final, you could create a subclass and have two strings that look alike when “seen as Strings”, but that are actually different.