How to remove a task from ScheduledExecutorService?

Simply cancel the future returned by scheduledAtFixedRate():

// Create the scheduler
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1);
// Create the task to execute
Runnable r = new Runnable() {
    @Override
    public void run() {
        System.out.println("Hello");
    }
};
// Schedule the task such that it will be executed every second
ScheduledFuture<?> scheduledFuture =
    scheduledExecutorService.scheduleAtFixedRate(r, 1L, 1L, TimeUnit.SECONDS);
// Wait 5 seconds
Thread.sleep(5000L);
// Cancel the task
scheduledFuture.cancel(false);

Another thing to note is that cancel does not remove the task from scheduler. All it ensures is that isDone method always return true. This may lead to memory leaks if you keep adding such tasks. For e.g.: if you start a task based on some client activity or UI button click, repeat it n-times and exit. If that button is clicked too many times, you might end up with big pool of threads that cannot be garbage collected as scheduler still has a reference.

You may want to use setRemoveOnCancelPolicy(true) in ScheduledThreadPoolExecutor class available in Java 7 onwards. For backward compatibility, default is set to false.

If your ScheduledExecutorService instance extends ThreadPoolExecutor (e.g. ScheduledThreadPoolExecutor), you could use remove(Runnable) (but see the note in its javadoc: "It may fail to remove tasks that have been converted into other forms before being placed on the internal queue.") or purge().