code: public int addAndGet(T obj, int delta) {
          int prev, next;
          do {
              prev = get(obj);
              next = prev + delta;
          } while (!compareAndSet(obj, prev, next));
          return next;
      }
docs: Atomically adds the given value to the current value of the field of
  the given object managed by this updater.
  
  @param obj An object whose field to get and set
  @param delta the value to add
  @return the updated value