In Java, what is the impact of declaring a method as final in a class?

Introduction / Context:
The final keyword in Java can be applied to variables, methods, and classes, and it has a slightly different meaning in each context. For methods, using final is often discussed in interviews because it affects inheritance and polymorphism. Understanding the impact of final on methods is important when designing stable APIs and preventing unwanted changes in subclasses.

Given Data / Assumptions:

• There is a superclass that defines a method.
• The method is declared using the final keyword.
• There may be subclasses that extend this superclass.
• The designer of the superclass might want to protect certain behavior from modification.

Concept / Approach:
When a method is declared as final, it cannot be overridden by any subclass. This means that subclasses cannot provide their own implementation for that method. The original behavior defined in the superclass is preserved in all subclasses. This is often used for methods that implement critical logic which should remain consistent, such as security checks, transaction handling, or template method steps that must not be changed.

Step-by-Step Solution:
1. Define a superclass, for example class PaymentProcessor, and declare a method processPayment() as final. 2. When another class, such as CreditCardProcessor, extends PaymentProcessor, it inherits processPayment(). 3. Because processPayment() is final, CreditCardProcessor cannot override it with a different implementation. 4. If you attempt to declare a method in the subclass with the same signature and mark it as overriding processPayment(), the compiler produces an error. 5. The result is that the behavior of processPayment() remains exactly as defined in PaymentProcessor for all subclasses.

Verification / Alternative check:
You can verify the effect of final by writing a small test program. When you remove final, overriding becomes possible. When you add final back, attempts to override the method cause compile time errors. Note that final does not automatically make the method static, nor does it guarantee performance optimization. While some JVM implementations may inline final methods, this is an optimization detail and not part of the Java language contract.

Why Other Options Are Wrong:
Option B is wrong because declaring a method final does not prevent it from being called inside its own class; it only prevents overriding in subclasses. Option C is incorrect because final does not imply static; static methods are associated with the class, while final simply prevents overriding. Option D is misleading; although final methods are sometimes easier for the JVM to optimize, there is no guarantee in the language that they will always execute faster or be inlined.

Common Pitfalls:
A common mistake is to mark too many methods as final, which can reduce flexibility for future extensions. Another pitfall is thinking that final implies thread safety or immutability by itself; those concerns require additional design choices. Developers also sometimes confuse final with finally or finalize, which are entirely different concepts in Java related to exception handling and object cleanup. Clarity on the role of final for methods is essential for good API design.

Final Answer:
The method cannot be overridden in any subclass, which helps preserve its implementation and behavior.