code: /*
   * Copyright 2015 recommenders.net.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package net.recommenders.rival.core;
  
  /**
   * Interface for a temporal data model. It is able to store users, items,
   * preferences, and timestamps.
   *
   * @author <a href="http://github.com/abellogin">Alejandro</a>, <a
   * href="http://github.com/alansaid">Alan</a>
   *
   * @param <U> generic type for users
   * @param <I> generic type for items
   */
  public interface TemporalDataModelIF<U, I> extends DataModelIF<U, I> {
  
      /**
       * Method that returns the timestamps between a user and an item.
       *
       * @param u the user.
       * @param i the item.
       * @return the timestamps between a user and an item.
       */
      public Iterable<Long> getUserItemTimestamps(U u, I i);
  
      /**
       * Method that adds a timestamp to the model between a user and an item.
       *
       * @param u the user.
       * @param i the item.
       * @param t the timestamp.
       */
      public void addTimestamp(final U u, final I i, final Long t);
  }
  