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    import java.util.Iterator;
023    
024    import org.apache.hadoop.mapred.Reducer;
025    import org.apache.hadoop.mapred.OutputCollector;
026    import org.apache.hadoop.mapred.Reporter;
027    import org.apache.hadoop.mapred.MapReduceBase;
028    
029    import org.apache.hadoop.classification.InterfaceAudience;
030    import org.apache.hadoop.classification.InterfaceStability;
031    import org.apache.hadoop.io.LongWritable;
032    
033    /** 
034     * A {@link Reducer} that sums long values. 
035     */
036    @InterfaceAudience.Public
037    @InterfaceStability.Stable
038    public 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    }