code: public boolean putReference(K key, ServiceReference<V> reference) {
          if (key == null || reference == null)
              return false;
  
          ConcurrentServiceReferenceElement<V> element = new ConcurrentServiceReferenceElement<V>(referenceName, reference);
          return (elementMap.put(key, element) != null);
      }
docs: Associates the reference with the key.
  
  @param key Key associated with this reference
  @param reference ServiceReference for the target service
  @return true if this is replacing a previous (non-null) service reference