BinaryPartitioner (Hadoop 0.20.2-cdh3u4 API)
Date: 2012-05-07

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## org.apache.hadoop.mapreduce.lib.partition
Class BinaryPartitioner<V>

    java.lang.Object
      org.apache.hadoop.mapreduce.Partitioner<BinaryComparable,V>
          org.apache.hadoop.mapreduce.lib.partition.BinaryPartitioner<V>

All Implemented Interfaces:
  Configurable

Direct Known Subclasses:
  BinaryPartitioner

    public class BinaryPartitioner<V>

extends Partitioner<BinaryComparable,V>
implements Configurable

Partition `BinaryComparable` keys using a configurable part of the bytes array returned by `BinaryComparable.getBytes()`.

The subarray to be used for the partitioning can be defined by means of the following properties:

- mapred.binary.partitioner.left.offset: left offset in array (0 by default)
- mapred.binary.partitioner.right.offset: right offset in array (-1 by default)

Like in Python, both negative and positive offsets are allowed, but the meaning is slightly different. In case of an array of length 5, for instance, the possible offsets are:

      +---+---+---+---+---+
      | B | B | B | B | B |
      +---+---+---+---+---+
        0   1   2   3   4
       -5  -4  -3  -2  -1

The first row of numbers gives the position of the offsets 0...5 in the array; the second row gives the corresponding negative offsets. Contrary to Python, the specified subarray has byte `i` and `j` as first and last element, repectively, when `i` and `j` are the left and right offset.

For Hadoop programs written in Java, it is advisable to use one of the following static convenience methods for setting the offsets:

- `setOffsets(org.apache.hadoop.conf.Configuration, int, int)`
- `setLeftOffset(org.apache.hadoop.conf.Configuration, int)`
- `setRightOffset(org.apache.hadoop.conf.Configuration, int)`

Constructor Summary
`BinaryPartitioner()`

Method Summary
`Configuration` | `getConf()` Return the configuration used by this object.
`int` | `getPartition(BinaryComparable key, V value, int numPartitions)` Use (the specified slice of the array returned by) `BinaryComparable.getBytes()` to partition.
`void` | `setConf(Configuration conf)` Set the configuration to be used by this object.
`static void` | `setLeftOffset(Configuration conf, int offset)` Set the subarray to be used for partitioning to `bytes[offset:]` in Python syntax.
`static void` | `setOffsets(Configuration conf, int left, int right)` Set the subarray to be used for partitioning to `bytes[left:(right+1)]` in Python syntax.
`static void` | `setRightOffset(Configuration conf, int offset)` Set the subarray to be used for partitioning to `bytes[:(offset+1)]` in Python syntax.

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

Constructor Detail

### BinaryPartitioner

    public BinaryPartitioner()

Method Detail

### setOffsets

    public static void setOffsets(Configuration conf,
                                  int left,
                                  int right)

  Set the subarray to be used for partitioning to `bytes[left:(right+1)]` in Python syntax.

  Parameters:
    `conf` - configuration object
    `left` - left Python-style offset
    `right` - right Python-style offset

### setLeftOffset

    public static void setLeftOffset(Configuration conf,
                                     int offset)

  Set the subarray to be used for partitioning to `bytes[offset:]` in Python syntax.

  Parameters:
    `conf` - configuration object
    `offset` - left Python-style offset

### setRightOffset

    public static void setRightOffset(Configuration conf,
                                      int offset)

  Set the subarray to be used for partitioning to `bytes[:(offset+1)]` in Python syntax.

  Parameters:
    `conf` - configuration object
    `offset` - right Python-style offset

### setConf

    public void setConf(Configuration conf)

  Description copied from interface: `Configurable`
  Set the configuration to be used by this object.

  Specified by:
    `setConf` in interface `Configurable`

### getConf

    public Configuration getConf()

  Description copied from interface: `Configurable`
  Return the configuration used by this object.

  Specified by:
    `getConf` in interface `Configurable`

### getPartition

    public int getPartition(BinaryComparable key,
                            V value,
                            int numPartitions)

  Use (the specified slice of the array returned by) `BinaryComparable.getBytes()` to partition.

  Specified by:
    `getPartition` in class `Partitioner<BinaryComparable,V>`
  Parameters:
    `key` - the key to be partioned.
    `value` - the entry value.
    `numPartitions` - the total number of partitions.
  Returns:
    the partition number for the `key`.

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