001/*
002 * Copyright (c) 2007-2015 Concurrent, Inc. All Rights Reserved.
003 *
004 * Project and contact information: http://www.cascading.org/
005 *
006 * This file is part of the Cascading project.
007 *
008 * Licensed under the Apache License, Version 2.0 (the "License");
009 * you may not use this file except in compliance with the License.
010 * You may obtain a copy of the License at
011 *
012 *     http://www.apache.org/licenses/LICENSE-2.0
013 *
014 * Unless required by applicable law or agreed to in writing, software
015 * distributed under the License is distributed on an "AS IS" BASIS,
016 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
017 * See the License for the specific language governing permissions and
018 * limitations under the License.
019 */
020
021package cascading.pipe.joiner;
022
023import java.io.Serializable;
024import java.util.Iterator;
025
026import cascading.tuple.Tuple;
027
028/**
029 * Interface Joiner allows for custom join strategies against a {@link cascading.pipe.CoGroup}.
030 * <p/>
031 * Joins perform based on the equality of the join keys. In the case of null values, Java treats two
032 * null values as equivalent. SQL does not treat null values as equal. To produce SQL like results in a given
033 * join, a new {@link java.util.Comparator} will need to be used on the joined values to prevent null from
034 * equaling null. As a convenience, see the {@link cascading.util.NullNotEquivalentComparator} class.
035 */
036public interface Joiner extends Serializable
037  {
038  /**
039   * Returns an iterator that joins the given CoGroupClosure co-groups.
040   *
041   * @param closure of type GroupClosure
042   * @return an iterator
043   */
044  Iterator<Tuple> getIterator( JoinerClosure closure );
045
046  /**
047   * Returns the number of joins this instance can handle. A value of -1 denotes there is no limit.
048   *
049   * @return an int
050   */
051  int numJoins();
052  }