code: public ServiceReference<V> getReference(K key) {
          if (key == null)
              return null;
  
          ConcurrentServiceReferenceElement<V> e = elementMap.get(key);
          if (e == null) {
              return null;
          } else {
              return e.getReference();
          }
      }
docs: Returns the ServiceReference associated with key
  
  @param key The key associated with the service
  @return ServiceRerefence associated with key, or null