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
019package org.apache.hadoop.mapred.lib;
020
021import java.io.IOException;
022
023import java.util.Iterator;
024
025import org.apache.hadoop.classification.InterfaceAudience;
026import org.apache.hadoop.classification.InterfaceStability;
027import org.apache.hadoop.mapred.Reducer;
028import org.apache.hadoop.mapred.OutputCollector;
029import org.apache.hadoop.mapred.Reporter;
030import 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
037public 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}