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    
021    package cascading.pipe.joiner;
022    
023    import java.io.Serializable;
024    import java.util.Iterator;
025    
026    import 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     */
036    public 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      }