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Hadoop is the distributed computing framework of Apache; hadoop-core contains the filesystem, job tracker and map/reduce modules

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package org.apache.hadoop.mapreduce;

/** 
 * 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 Reducer */ public abstract class Partitioner { /** * 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.

* * @param key the key to be partioned. * @param value the entry value. * @param numPartitions the total number of partitions. * @return the partition number for the key. */ public abstract int getPartition(KEY key, VALUE value, int numPartitions); }




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