code: private Map<String, RelationshipResult> getCache(final Direction direction) {
  
  		final String directionKey             = direction != null ? direction.name() : "*";
  		Map<String, RelationshipResult> cache = relationshipCache.get(directionKey);
  
  		if (cache == null) {
  
  			cache = new HashMap<>();
  			relationshipCache.put(directionKey, cache);
  		}
  
  		return cache;
  	}
docs: ----- private methods -----