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7Partitioner (Hadoop 0.20.1 API)
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91org.apache.hadoop.mapreduce</FONT>
92<BR>
93Class Partitioner&lt;KEY,VALUE&gt;</H2>
94<PRE>
95<A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">java.lang.Object</A>
96  <IMG SRC="../../../../resources/inherit.gif" ALT="extended by "><B>org.apache.hadoop.mapreduce.Partitioner&lt;KEY,VALUE&gt;</B>
97</PRE>
98<DL>
99<DT><B>Direct Known Subclasses:</B> <DD><A HREF="../../../../org/apache/hadoop/mapreduce/lib/partition/HashPartitioner.html" title="class in org.apache.hadoop.mapreduce.lib.partition">HashPartitioner</A>, <A HREF="../../../../org/apache/hadoop/examples/SecondarySort.FirstPartitioner.html" title="class in org.apache.hadoop.examples">SecondarySort.FirstPartitioner</A></DD>
100</DL>
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102<DL>
103<DT><PRE>public abstract class <B>Partitioner&lt;KEY,VALUE&gt;</B><DT>extends <A HREF="http://java.sun.com/javase/6/docs/api/java/lang/Object.html?is-external=true" title="class or interface in java.lang">Object</A></DL>
104</PRE>
105
106<P>
107Partitions the key space.
108 
109 <p><code>Partitioner</code> controls the partitioning of the keys of the
110 intermediate map-outputs. The key (or a subset of the key) is used to derive
111 the partition, typically by a hash function. The total number of partitions
112 is the same as the number of reduce tasks for the job. Hence this controls
113 which of the <code>m</code> reduce tasks the intermediate key (and hence the
114 record) is sent for reduction.</p>
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116
117<P>
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119<DT><B>See Also:</B><DD><A HREF="../../../../org/apache/hadoop/mapreduce/Reducer.html" title="class in org.apache.hadoop.mapreduce"><CODE>Reducer</CODE></A></DL>
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133<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html#Partitioner()">Partitioner</A></B>()</CODE>
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150<CODE>abstract &nbsp;int</CODE></FONT></TD>
151<TD><CODE><B><A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html#getPartition(KEY, VALUE, int)">getPartition</A></B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html" title="type parameter in Partitioner">KEY</A>&nbsp;key,
152             <A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html" title="type parameter in Partitioner">VALUE</A>&nbsp;value,
153             int&nbsp;numPartitions)</CODE>
154
155<BR>
156&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Get the partition number for a given key (hence record) given the total
157 number of partitions i.e.</TD>
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181
182<A NAME="Partitioner()"><!-- --></A><H3>
183Partitioner</H3>
184<PRE>
185public <B>Partitioner</B>()</PRE>
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198
199<A NAME="getPartition(java.lang.Object,java.lang.Object,int)"><!-- --></A><A NAME="getPartition(KEY, VALUE, int)"><!-- --></A><H3>
200getPartition</H3>
201<PRE>
202public abstract int <B>getPartition</B>(<A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html" title="type parameter in Partitioner">KEY</A>&nbsp;key,
203                                 <A HREF="../../../../org/apache/hadoop/mapreduce/Partitioner.html" title="type parameter in Partitioner">VALUE</A>&nbsp;value,
204                                 int&nbsp;numPartitions)</PRE>
205<DL>
206<DD>Get the partition number for a given key (hence record) given the total
207 number of partitions i.e. number of reduce-tasks for the job.
208   
209 <p>Typically a hash function on a all or a subset of the key.</p>
210<P>
211<DD><DL>
212<DT><B>Parameters:</B><DD><CODE>key</CODE> - the key to be partioned.<DD><CODE>value</CODE> - the entry value.<DD><CODE>numPartitions</CODE> - the total number of partitions.
213<DT><B>Returns:</B><DD>the partition number for the <code>key</code>.</DL>
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