null value output from map...

[Andy Sautins]

 

   In writing a Map/Reduce job I ran across something I found a little
strange.  I have a situation where I don't need a value output from map.
If I set the value of the value of OutputCollector<Text, IntWritable> to
null I get the following exception:

 

java.lang.NullPointerException

   at
org.apache.hadoop.mapred.MapTask$MapOutputBuffer.collect(MapTask.java:56
2)

 

    Looking at the code in MapTask.java ( Hadoop .19.1 ) it makes sense
why it would throw the exception:

 

      if (value.getClass() != valClass) {

        throw new IOException("Type mismatch in value from map: expected
"

                              + valClass.getName() + ", recieved "

                              + value.getClass().getName());

      }

 

  I guess my question is as follows: is it a bad idea/not normal to
collect a null value in map?  Outputting from reduce through
TextOutputFormat with a null value as I expect.  If the value is null
only they key and newline are output.  

 

   Any thoughts would be appreciated.