protected RelationshipView addRelationship(Relationship relationship) { if (relationship == null) { throw new IllegalArgumentException("A relationship must be specified."); } if (isElementInView(relationship.getSource()) && isElementInView(relationship.getDestination())) { RelationshipView relationshipView = new RelationshipView(relationship); relationshipViews.add(relationshipView); return relationshipView; } return null; }