title: Key of object type in the hadoop mapper
body: <p>New to hadoop and trying to understand the mapreduce wordcount example code from <a href="http://hadoop.apache.org/docs/current/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html" rel="noreferrer">here</a>. </p>
  
  <p>The mapper from documentation is - </p>
  
  <pre><code>Mapper&lt;KEYIN,VALUEIN,KEYOUT,VALUEOUT&gt;
  </code></pre>
  
  <p>I see that in the mapreduce word count example the map code is as follows</p>
  
  <pre><code>public void map(Object key, Text value, Context context)
  </code></pre>
  
  <p>Question - What is the point of this key of type Object? If the input to a mapper is a text document I am assuming the value in would be the chunk of text (64MB or 128MB) that hadoop has partitioned and stored in HDFS.  <strong>More generally, what is the use of this input key Keyin to the map code?</strong></p>
  
  <p>Any pointers would be greatly appreciated</p>