Since String objects are cached in a String pool, it makes sense to make the String immutable. The cached String literals are shared between multiple clients. And there is a possibility that one client's action may affect another client’s access to String pool.

String is also used as a parameter in many Java classes. Eg. You can pass hostname, port number as String while opening a network connection. If any one can modify your copy of the String, it can change the hostname. Due to this reason, it makes sense to make String final as soon as it is created.