title: Purpose Of NullWritable
body: <p>I want to count the number of studens who have booked a movie ticket and I wannt only one output after the reduce phase. I desire that the mapper emits the count of the number of students not the keys.
  Can I use NullWritable as an Output Key so that nothing is emitted from the map side as the key to the reduce side? 
  as shown below</p>
  
  <pre><code>context.write(NullWritable.get(),new IntWritable(1);
  </code></pre>
  
  <p>The data will be emitted to the reducer and the reducer will perform further aggregation</p>
  
  <p>Please suggest if anybody has a better alternative.</p>
  
  <p>Thank You in advance!!</p>