What is the difference between Runnable and Callable, and where does Future fit?

Runnable.run() returns void and cannot throw checked exceptions, so a failure inside it has nowhere to go except the thread's uncaught exception handler. Callable.call() returns V and may throw, which is what lets an executor capture the outcome.

submit wraps either into a Future. Calling get() blocks until the task finishes and then either returns the value or throws ExecutionException with the original failure as its cause. That is the important half: with execute(Runnable) an exception is printed and lost, with submit it is stored in the Future and is lost only if nobody ever calls get().