Design Phone Directory

Design a Phone Directory which supports the following operations:

get: Provide a number which is not assigned to anyone.
check: Check if a number is available or not.
release: Recycle or release a number.

 public class PhoneDirectory {
     Set<Integer> used = new HashSet<>();
     Queue<Integer> available = new LinkedList<>();
     int max;

     public PhoneDirectory(int maxNumbers) {
         max = maxNumbers;
         for (int i = ; i < maxNumbers; i++) {
             available.offer(i);
         }
     }

     public int get() {
         Integer ret = available.poll();
         if (ret == null) {
             return -;
         }
         used.add(ret);
         return ret;
     }

     public boolean check(int number) {
         if (number >= max || number < ) {
             return false;
         }
         return !used.contains(number);
     }

     public void release(int number) {
         if (used.remove(number)) {
             available.offer(number);
         }
     }
 }