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 different meaning in each context. When used with methods, final is particularly important in inheritance hierarchies because it controls whether subclasses can change the implementation. Interview questions often ask about the impact of final on methods to ensure that you understand the rules of overriding, overloading, and class design for extensibility.

Given Data / Assumptions:

• We are working with instance or static methods of a Java class.
• The method is declared with the final modifier.
• We consider subclasses that might otherwise attempt to override the method.
• Overloading (same method name, different parameter list) remains a separate concept from overriding.

Concept / Approach:
Declaring a method as final in Java means that no subclass can override that method. The implementation in the declaring class is fixed for all subclasses, which can be useful to preserve important invariants or security checks. However, the method can still be overloaded in the same class or in subclasses by providing additional methods with the same name but different parameter lists. Final does not change the access level or make the method static; it simply prevents overriding. Performance improvements from final methods are possible but are handled by the compiler and JVM and are not guaranteed by the language specification.

Step-by-Step Solution:
Step 1: Recall that overriding occurs when a subclass provides a method with the same signature (name and parameter types) as a method in its superclass. Step 2: Understand that marking a method final tells the compiler that this method must not be overridden in any subclass. Step 3: Recognise that overloading is different: methods with the same name but different parameter lists can coexist, even if one of them is final. Step 4: Note that final does not automatically restrict who can call the method; access is still determined by public, protected, or private. Step 5: Match this behaviour to the option stating that the method cannot be overridden but can still be overloaded in the same class.

Verification / Alternative check:
You can test this by writing a base class with a public final void doWork() method and then trying to override doWork() in a subclass. The compiler will report an error indicating that you cannot override a final method. If you instead create a method doWork(int x) in the same class or subclass, this compiles successfully because it is an overload, not an override. This experiment confirms that final prevents overriding but does not prohibit overloading or change static behaviour.

Why Other Options Are Wrong:
Option It cannot be called from outside the class: This confuses final with private; access is controlled by visibility modifiers, not by final. Option It becomes a static method that belongs only to the class: Static and final are unrelated concepts; final methods can be either instance or static methods. Option It causes the method to run faster by default, regardless of the compiler: While compilers may optimise final methods, the Java language specification does not guarantee performance improvements simply because a method is final.

Common Pitfalls:
One pitfall is to overuse final on methods, which can make a class difficult to extend or mock for testing. Another is to believe that finals are always inlined or optimised, leading to premature micro optimisation. Some developers also confuse final with immutable, forgetting that final on a method does not make objects immutable; it just prevents overriding. In interview answers, focus on the clear rule: final methods cannot be overridden by subclasses, but they can still be overloaded and called according to their access level.

Final Answer:
Declaring a method as final means It cannot be overridden in any subclass, but it can still be overloaded in the same class.