Which keyword is used to prevent a class from being subclassed in Java?

• A. static
• B. final
• C. private
• D. abstract

Answer: (B)

The keyword that is used to prevent a class from being subclassed in Java is 'final'. When a class is declared as final, it cannot be extended by any other class. This is particularly useful when you want to create an immutable class or ensure that the behavior of the class remains unchanged. By preventing subclassing, the final class guarantees that the intended functionality is preserved, without the risk of modification through inheritance.

For example, if you have a class designed to handle specific operations, making it final ensures that no other class can alter its inherited behavior, thus maintaining its integrity and security. Furthermore, this can potentially lead to performance improvements, as the Java Virtual Machine can optimize method calls when it knows a class will not be subclassed.

This makes the final keyword an important tool in Java for controlling class design and inheritance, especially in scenarios where stability and reliability are paramount.