Does the class of the Mapper output need to match the exact class of the specified output?

[Wilkes, Chris]

I'm outputting a Text and a LongWritable in my mapper and told the job  
that my mapper output class is Writable (the interface shared by both  
of them):
  job.setMapOutputValueClass(Writable.class);
I'm doing this as I have two different types of input files and am  
combining them together.  I could write them both as as Text but then  
I'll have to put a marker in front of the tag to indicate what type of  
entry it is instead of doing a
  if (value instanceof Text) { }  else if (value instanceof  
LongWritable) { }

This exception is thrown:

java.io.IOException: Type mismatch in value from map: expected  
org.apache.hadoop.io.Writable, recieved  
org.apache.hadoop.io.LongWritable
	at org.apache.hadoop.mapred.MapTask 
$MapOutputBuffer.collect(MapTask.java:812)
	at org.apache.hadoop.mapred.MapTask 
$NewOutputCollector.write(MapTask.java:504)
	at  
org 
.apache 
.hadoop 
.mapreduce.TaskInputOutputContext.write(TaskInputOutputContext.java:80)
The MapTask code (which is being used even though I'm using the new  
API) shows that a != is used to compare the classes:
http://svn.apache.org/viewvc/hadoop/mapreduce/trunk/src/java/org/apache/hadoop/mapred/MapTask.java?view=log

 if (value.getClass() != valClass) {
        throw new IOException("Type mismatch in value from map:  
expected "
                              + valClass.getName() + ", recieved "
                              + value.getClass().getName());
      }

Does this level of checking really need to be done?  Could it just be  
a Class.isAssignableFrom() check?