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An immutable library value such as java.lang.String can be a final class in Java. No class can extend it, so a client receiving a String cannot secretly receive a subclass that changes the behavior of its methods. This protects assumptions about value semantics in code using strings as keys or trusted identifiers.

The benefit is a closed implementation contract. Without final, subclasses might override methods inconsistently or introduce behavior incompatible with the library's intended guarantees. Making a class final does not by itself make its fields immutable; a correct immutable implementation also controls its state. The cost is that clients cannot customize it by inheritance and must instead use composition or a different abstraction.