Class LongSumReducer<K>

  extended by org.apache.hadoop.mapred.MapReduceBase
      extended by org.apache.hadoop.mapred.lib.LongSumReducer<K>
All Implemented Interfaces:
Closeable, JobConfigurable, Reducer<K,LongWritable,K,LongWritable>

public class LongSumReducer<K>
extends MapReduceBase
implements Reducer<K,LongWritable,K,LongWritable>

A Reducer that sums long values.

Constructor Summary
Method Summary
 void reduce(K key, Iterator<LongWritable> values, OutputCollector<K,LongWritable> output, Reporter reporter)
          Reduces values for a given key.
Methods inherited from class org.apache.hadoop.mapred.MapReduceBase
close, configure
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Methods inherited from interface org.apache.hadoop.mapred.JobConfigurable
Methods inherited from interface

Constructor Detail


public LongSumReducer()
Method Detail


public void reduce(K key,
                   Iterator<LongWritable> values,
                   OutputCollector<K,LongWritable> output,
                   Reporter reporter)
            throws IOException
Description copied from interface: Reducer
Reduces values for a given key.

The framework calls this method for each <key, (list of values)> pair in the grouped inputs. Output values must be of the same type as input values. Input keys must not be altered. The framework will reuse the key and value objects that are passed into the reduce, therefore the application should clone the objects they want to keep a copy of. In many cases, all values are combined into zero or one value.

Output pairs are collected with calls to OutputCollector.collect(Object,Object).

Applications can use the Reporter provided to report progress or just indicate that they are alive. In scenarios where the application takes an insignificant amount of time to process individual key/value pairs, this is crucial since the framework might assume that the task has timed-out and kill that task. The other way of avoiding this is to set mapred.task.timeout to a high-enough value (or even zero for no time-outs).

Specified by:
reduce in interface Reducer<K,LongWritable,K,LongWritable>
key - the key.
values - the list of values to reduce.
output - to collect keys and combined values.
reporter - facility to report progress.

Copyright © 2009 The Apache Software Foundation