Final answer:
A final class is a class that cannot be subclassed, a final method cannot be overridden, and a final variable cannot have its value changed after assignment. They are used to maintain integrity and security of code behavior and to allow compiler optimizations.
Step-by-step explanation:
A final class in object-oriented programming languages like Java is a class that cannot be subclassed or extended. Similarly, a final method is a function that cannot be overridden by subclasses, ensuring that the functionality defined by the method is preserved and unchangeable in any inheriting classes. A final variable, once assigned a value, cannot be changed; it effectively becomes a constant. We utilize final classes, methods, and variables to secure the behavior of code against alteration, maintain the integrity of the implementation, and sometimes to enhance performance optimizations made by the compiler.