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;
022import java.util.Iterator;
023
024import org.apache.hadoop.mapred.Reducer;
025import org.apache.hadoop.mapred.OutputCollector;
026import org.apache.hadoop.mapred.Reporter;
027import org.apache.hadoop.mapred.MapReduceBase;
028
029import org.apache.hadoop.classification.InterfaceAudience;
030import org.apache.hadoop.classification.InterfaceStability;
031import org.apache.hadoop.io.LongWritable;
032
033/** 
034 * A {@link Reducer} that sums long values. 
035 */
036@InterfaceAudience.Public
037@InterfaceStability.Stable
038public class LongSumReducer<K> extends MapReduceBase
039    implements Reducer<K, LongWritable, K, LongWritable> {
040
041  public void reduce(K key, Iterator<LongWritable> values,
042                     OutputCollector<K, LongWritable> output,
043                     Reporter reporter)
044    throws IOException {
045
046    // sum all values for this key
047    long sum = 0;
048    while (values.hasNext()) {
049      sum += values.next().get();
050    }
051
052    // output sum
053    output.collect(key, new LongWritable(sum));
054  }
055
056}