The difference between Callable and Runnable is one of the most frequently asked multi-threading and concurrency interview questions in the Java world. I remember it was 2007 when I first heard about the Callable interface and that too on a telephonic interview. Till then, I was happy using Runnable to implement threads and just started paying attention to Java 1.5, as most of the applications by then using Java 1.4. That one interview question encouraged me to learn more about several other useful features introduced in Java 5 concurrency library like CountDownLatch, CyclicBarrier, Semaphore, Atomic variables, and Thread pool. This is one of the reasons I always encourage Java developers to give/take regular interviews, just to update your knowledge.