org.apache.hadoop.mapred
Interface Reducer

All Superinterfaces:

Closeable, JobConfigurable

All Known Implementing Classes:

DataJoinMapperBase, DataJoinReducerBase, FieldSelectionMapReduce, IdentityReducer, JobBase, LongSumReducer, PiEstimator.PiReducer, PipeReducer, ValueAggregatorCombiner, ValueAggregatorJobBase, ValueAggregatorMapper, ValueAggregatorReducer, WordCount.Reduce

public interface Reducer
extends JobConfigurable, Closeable

Reduces a set of intermediate values which share a key to a smaller set of values. Input values are the grouped output of a Mapper.

Method Summary

void	reduce(WritableComparable key, Iterator values, OutputCollector output, Reporter reporter) Combines values for a given key.

Methods inherited from interface org.apache.hadoop.mapred.JobConfigurable

configure

Methods inherited from interface org.apache.hadoop.io.Closeable

close

Method Detail

reduce

void reduce(WritableComparable key,
            Iterator values,
            OutputCollector output,
            Reporter reporter)
            throws IOException

Combines values for a given key. Output values must be of the same type as input values. Input keys must not be altered. Typically all values are combined into zero or one value. Output pairs are collected with calls to OutputCollector.collect(WritableComparable,Writable).

Parameters:

key - the key

values - the values to combine

output - to collect combined values

Throws:

IOException