public class SafeBox<V> {
  private V value;
  private final Monitor monitor = new Monitor();
  private final Monitor.Guard valuePresent = guard(monitor, () -> value != null);
  private final Monitor.Guard valueAbsent = guard(monitor, () -> value == null);

  public V get() throws InterruptedException {
    try (LockedMonitor ignored = monitor.autoEnterWhen(valuePresent)) {
      V result = value;
      value = null;
      return result;
    }
  }

  public void set(V newValue) throws InterruptedException {
    try (LockedMonitor ignored = monitor.autoEnterWhen(valueAbsent)) {
      value = newValue;
    }
  }
}