private void addNewExecutorIfNecessary() {
Set<Integer> availableNumbers = new HashSet<Integer>();
for (int i = 0; i < numExecutors; i++) availableNumbers.add(i);
for (Executor executor : executors) availableNumbers.remove(executor.getNumber());
for (Integer number : availableNumbers) {
Executor e = new Executor(this, number);
e.start();
executors.add(e);
}
}
