## org.apache.hadoop.mapreduce
Class Partitioner<KEY,VALUE>

    java.lang.Object
      org.apache.hadoop.mapreduce.Partitioner<KEY,VALUE>

Direct Known Subclasses:
  HashPartitioner, SecondarySort.FirstPartitioner

    public abstract class Partitioner<KEY,VALUE>

extends Object

Partitions the key space.

`Partitioner` controls the partitioning of the keys of the intermediate map-outputs. The key (or a subset of the key) is used to derive the partition, typically by a hash function. The total number of partitions is the same as the number of reduce tasks for the job. Hence this controls which of the `m` reduce tasks the intermediate key (and hence the record) is sent for reduction.

See Also:
  `Reducer`

Constructor Summary
`Partitioner()`

Method Summary
`abstract int` `getPartition(KEY key, VALUE value, int numPartitions)` Get the partition number for a given key (hence record) given the total number of partitions i.e.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### Partitioner

    public Partitioner()

Method Detail

### getPartition

    public abstract int getPartition(KEY key,
                                     VALUE value,
                                     int numPartitions)

  Get the partition number for a given key (hence record) given the total number of partitions i.e. number of reduce-tasks for the job.

  Typically a hash function on a all or a subset of the key.

  Parameters:
    `key` - the key to be partioned.
    `value` - the entry value.
    `numPartitions` - the total number of partitions.
  Returns:
    the partition number for the `key`.

Copyright © 2009 The Apache Software Foundation