org.apache.hadoop.examples
Class WordCount.Reduce

java.lang.Object
  org.apache.hadoop.mapred.MapReduceBase
      org.apache.hadoop.examples.WordCount.Reduce

All Implemented Interfaces:

Closeable, JobConfigurable, Reducer<Text,IntWritable,Text,IntWritable>

Enclosing class:

WordCount

public static class WordCount.Reduce
extends MapReduceBase
implements Reducer<Text,IntWritable,Text,IntWritable>

A reducer class that just emits the sum of the input values.

Constructor Summary

WordCount.Reduce()

Method Summary

void	reduce(Text key, Iterator<IntWritable> values, OutputCollector<Text,IntWritable> 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

configure

Methods inherited from interface java.io.Closeable

close

Constructor Detail

WordCount.Reduce

public WordCount.Reduce()

Method Detail

reduce

public void reduce(Text key,
                   Iterator<IntWritable> values,
                   OutputCollector<Text,IntWritable> 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<Text,IntWritable,Text,IntWritable>

Parameters:

key - the key.

values - the list of values to reduce.

output - to collect keys and combined values.

reporter - facility to report progress.

Throws:

IOException