## Uses of Interface
org.apache.hadoop.mapred.Reducer

- Packages that use Reducer
  Package | Description
  --- | ---
  org.apache.hadoop.mapred |
  org.apache.hadoop.mapred.lib |
  org.apache.hadoop.mapred.lib.aggregate |

- - ### Uses of Reducer in org.apache.hadoop.mapred

    Methods in org.apache.hadoop.mapred that return types with arguments of type Reducer
    Modifier and Type | Method and Description
    --- | ---
    `Class<? extends Reducer>` | JobConf.`getCombinerClass()` Get the user-defined combiner class used to combine map-outputs before being sent to the reducers.
    `Class<? extends Reducer>` | JobConf.`getReducerClass()` Get the `Reducer` class for the job.

    Method parameters in org.apache.hadoop.mapred with type arguments of type Reducer
    Modifier and Type | Method and Description
    --- | ---
    `void` | JobConf.`setCombinerClass(Class<? extends Reducer> theClass)` Set the user-defined combiner class used to combine map-outputs before being sent to the reducers.
    `void` | JobConf.`setReducerClass(Class<? extends Reducer> theClass)` Set the `Reducer` class for the job.

  - ### Uses of Reducer in org.apache.hadoop.mapred.lib

    Classes in org.apache.hadoop.mapred.lib that implement Reducer
    Modifier and Type | Class and Description
    --- | ---
    `class` | `ChainReducer` The ChainReducer class allows to chain multiple Mapper classes after a Reducer within the Reducer task.
    `class` | `FieldSelectionMapReduce<K,V>` This class implements a mapper/reducer class that can be used to perform field selections in a manner similar to unix cut.
    `class` | `IdentityReducer<K,V>` Performs no reduction, writing all input values directly to the output.
    `class` | `LongSumReducer<K>` A `Reducer` that sums long values.

    Method parameters in org.apache.hadoop.mapred.lib with type arguments of type Reducer
    Modifier and Type | Method and Description
    --- | ---
    `static <K1,V1,K2,V2> void` | ChainReducer.`setReducer(JobConf job, Class<? extends Reducer<K1,V1,K2,V2>> klass, Class<? extends K1> inputKeyClass, Class<? extends V1> inputValueClass, Class<? extends K2> outputKeyClass, Class<? extends V2> outputValueClass, boolean byValue, JobConf reducerConf)` Sets the Reducer class to the chain job's JobConf.

  - ### Uses of Reducer in org.apache.hadoop.mapred.lib.aggregate

    Classes in org.apache.hadoop.mapred.lib.aggregate that implement Reducer
    Modifier and Type | Class and Description
    --- | ---
    `class` | `ValueAggregatorCombiner<K1 extends WritableComparable,V1 extends Writable>` This class implements the generic combiner of Aggregate.
    `class` | `ValueAggregatorJobBase<K1 extends WritableComparable,V1 extends Writable>` This abstract class implements some common functionalities of the the generic mapper, reducer and combiner classes of Aggregate.
    `class` | `ValueAggregatorMapper<K1 extends WritableComparable,V1 extends Writable>` This class implements the generic mapper of Aggregate.
    `class` | `ValueAggregatorReducer<K1 extends WritableComparable,V1 extends Writable>` This class implements the generic reducer of Aggregate.

Copyright © 2022 Apache Software Foundation. All rights reserved.