mapreduce/src/java/org/apache/hadoop/mapred/lib/LongSumReducer.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.hadoop.mapred.lib;

import java.io.IOException;
import java.util.Iterator;

import org.apache.hadoop.mapred.Reducer;
import org.apache.hadoop.mapred.OutputCollector;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.mapred.MapReduceBase;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.io.LongWritable;

/** A {@link Reducer} that sums long values. 
 */
@InterfaceAudience.Public
@InterfaceStability.Stable
public class LongSumReducer<K> extends MapReduceBase
    implements Reducer<K, LongWritable, K, LongWritable> {

  public void reduce(K key, Iterator<LongWritable> values,
                     OutputCollector<K, LongWritable> output,
                     Reporter reporter)
    throws IOException {

    // sum all values for this key
    long sum = 0;
    while (values.hasNext()) {
      sum += values.next().get();
    }

    // output sum
    output.collect(key, new LongWritable(sum));
  }

}