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--- courses:mapreduce-tutorial:step-24 [2012/01/27 21:28]
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+++ courses:mapreduce-tutorial:step-24 [2012/01/27 21:47]
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@@ Line 1: / Line 1: @@
 ====== MapReduce Tutorial : Mappers, running Java Hadoop jobs ======
-We start by exploring a simple Hadoop job with Mapper only. The Mapper outputs only keys starting with ''A''.
+We start by going through a simple Hadoop job with Mapper only.
+A mapper which processes (key, value) pairs of types (Kin, Vin) and produces (key, value) pairs of types (Kout, Vout) must be a subclass of [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/Mapper.html|Mapper<Kin, Vin, Kout, Vout>]]. In our case, the mapper is subclass of ''Mapper<Text, Text, Text, Text>''.
+The mapper must define a ''map'' method and may provide ''setup'' and ''context'' method:
+<code java>
+  public static class TheMapper extends Mapper<Text, Text, Text, Text>{
+    public void setup(Context context) throws IOException, InterruptedException {}
+    public void map(Text key, Text value, Context context) throws IOException, InterruptedException {}
+    public void cleanup(Context context) throws IOException, InterruptedException {}
+  }
+</code>
+Outputting (key, value) pairs is performed using the [[http://hadoop.apache.org/common/docs/r1.0.0/api/org/apache/hadoop/mapreduce/MapContext.html|MapContext<Kin, Vin, Kout, Vout>]] (the ''Context'' is an abbreviation for this type).
 <file java MapperOnlyHadoopJob.java>
 import java.io.IOException;

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