我是新来的 Apache Mahout . 我很困惑怎么办 GenericUserBasedRecommender 方法有效。例如：

UserSimilarity similarity =new PearsonCorrelationSimilarity (dataModel);  

UserNeighborhood neighborhood =new NearestNUserNeighborhood (2, similarity, dataModel);  

Recommender recommender = new GenericUserBasedRecommender (dataModel, neighborhood, similarity);  

Recommender cachingRecommender = new CachingRecommender(recommender);  

List<RecommendedItem> recommendations = cachingRecommender.recommend(12,10);

结果是：

user4 ­­­ 10.45  

user12 7.93  

user3 ­­­2.49

但是，如果我用 List<RecommendedItem> recommendations = cachingRecommender.recommend(12,5); 根本没有 recommendations .
所列建议决定了什么？有阈值吗？