What is the primary purpose of the `volatile` keyword in Java?

• A. To restrict a variable's access to one thread
• B. To ensure visibility of changes across threads
• C. To enhance performance by reducing memory usage
• D. To enforce immutability of a variable

Answer: (B)

The primary purpose of the volatile keyword in Java is to ensure visibility of changes across threads. When a variable is declared as volatile, it guarantees that any read of that variable will reflect the most recent write by any thread. This is crucial in multi-threaded environments where threads might be caching values of variables locally, leading to stale data being read if one thread modifies the variable.

By using volatile, changes made to the variable by one thread become immediately visible to other threads. This allows developers to avoid certain issues related to caching and synchronization that can arise when multiple threads are accessing a variable. For instance, if one thread updates a volatile variable, other threads will see this update right away, ensuring that they do not operate on outdated information.

This characteristic is particularly important in certain scenarios, such as implementing singleton patterns or managing flag states, where multiple threads need to access and modify shared variables without explicit synchronization. However, it’s important to note that while volatile ensures visibility and prevents caching, it does not provide atomicity for compound actions, which might still require additional synchronization mechanisms.