Java: Sealed Classes

It restricts class hierarchies to improve security and maintainability.

The 'sealed' keyword is crucial because it restricts the classes that can extend a given class, enhancing security and maintainability. By sealing a class, you prevent unintentional or unauthorized subclasses, which could introduce bugs or security risks. In a payment processing system, sealing a class like `PaymentMethod` ensures that only vetted methods are used, reducing the chance of errors.