001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    
019    package org.apache.hadoop.mapred.lib;
020    
021    import java.io.IOException;
022    
023    import java.util.Iterator;
024    
025    import org.apache.hadoop.classification.InterfaceAudience;
026    import org.apache.hadoop.classification.InterfaceStability;
027    import org.apache.hadoop.mapred.Reducer;
028    import org.apache.hadoop.mapred.OutputCollector;
029    import org.apache.hadoop.mapred.Reporter;
030    import org.apache.hadoop.mapred.MapReduceBase;
031    
032    /** 
033     * Performs no reduction, writing all input values directly to the output. 
034     */
035    @InterfaceAudience.Public
036    @InterfaceStability.Stable
037    public class IdentityReducer<K, V>
038        extends MapReduceBase implements Reducer<K, V, K, V> {
039    
040      /** Writes all keys and values directly to output. */
041      public void reduce(K key, Iterator<V> values,
042                         OutputCollector<K, V> output, Reporter reporter)
043        throws IOException {
044        while (values.hasNext()) {
045          output.collect(key, values.next());
046        }
047      }
048            
049    }