What is the difference between submit() and execute() on an ExecutorService?

execute(Runnable) simply runs the task and returns void, giving you no way to track when it finishes or to retrieve any exception it threw. submit(Runnable) instead returns a Future<?> that you can call get() on to block until the task completes and to discover whether it threw an exception; submitting a Callable<T> similarly returns a Future<T> that lets you retrieve the task's actual result. The key practical difference is exception handling: an unchecked exception thrown from a Runnable passed to execute() is silently reported to the thread's UncaughtExceptionHandler and never reaches the caller, whereas any exception thrown by a task submitted via submit() is captured inside its Future and re-thrown, wrapped, when you call get(). Because of this, submit() should generally be preferred in production code so failures aren't silently swallowed.