What happens if a method is declared both abstract and final?

• A. It can be implemented in a subclass
• B. It cannot have any body or implementation
• C. It causes a compilation error
• D. It can only exist in abstract classes

Answer: (C)

When a method is declared both abstract and final, it leads to a situation that defies the principles of object-oriented programming. An abstract method is intended to be overridden in a subclass, meaning that it does not have a body and must be implemented by any non-abstract subclass. On the other hand, a final method cannot be overridden in any subclass, which contradicts the purpose of an abstract method.

Given that the two modifiers contradict each other, the Java compiler will throw a compilation error when it encounters a method declared with both the abstract and final keywords. Therefore, no object can be created with such a method signature in a valid Java program, leading to a violation of the language's rules regarding method visibility and inheritance.

In contrast, the other options suggest scenarios that are not feasible under these conditions. An abstract method inherently cannot have an implementation (which would rule out a body), and while abstract methods can only exist within abstract classes, the final specification invalidates any implementation, thus making it impossible to have such a method in the first place. Thus, the conclusion is that declaring a method both abstract and final is inherently contradictory, resulting in a compilation error.