Runnable contract — which method signature must a class provide to correctly implement java.lang.Runnable?

Introduction / Context:
The Runnable interface defines the entry point executed by a Thread. Implementing classes must adhere to the exact method signature and visibility expected by the interface.

Given Data / Assumptions:

• Interface declaration is public interface Runnable { public abstract void run(); }
• Implementers must not reduce visibility on implemented methods.

Concept / Approach:
Since interface methods are implicitly public abstract, an implementing method must be declared public. Omitting public (package-private) would reduce visibility and cause a compile-time error.

Step-by-Step Solution:

Verification / Alternative check:
Try compiling a class with void run() (no public); the compiler reports that the method attempts to assign weaker access privileges than the interface method.

Why Other Options Are Wrong:
void run() (no public) has insufficient visibility; start() is a Thread method, not part of Runnable; overloads like run(int) do not satisfy the interface.

Common Pitfalls:
Forgetting to declare public or misspelling the signature.

Final Answer:
public void run()