Should mapreduce.ReduceContext reuse same object in nextKeyValue?

[Wilkes, Chris]

I created my own Writable class to store 3 pieces of information.  In  
my mapreducer.Reducer class I collect all of them and then process as  
a group, ie:

reduce(key, values, context) {
  List<Foo> myFoos =new ArrayList();
  for (Foo value : values) {
   myFoos.add(value);
  }
}
I was perplexed when entries in the list changed underneath me so I  
put a freeze() method on it:

boolean m_frozen = false;
public void freeze() {
  m_frozen = true;
}
@Override
public void readFields(DataInput in) throws IOException {       
  if (m_frozen) {
     throw new IllegalStateException();
  }
}

And noted that the exception was thrown:
        at Foo.readFields(Foo.java:169)
        at org.apache.hadoop.io.serializer.WritableSerialization 
$WritableDeserializer.deserialize(WritableSerialization.java:67)
        at org.apache.hadoop.io.serializer.WritableSerialization 
$WritableDeserializer.deserialize(WritableSerialization.java:40)
        at  
org 
.apache.hadoop.mapreduce.ReduceContext.nextKeyValue(ReduceContext.java: 
116)
        at org.apache.hadoop.mapreduce.ReduceContext 
$ValueIterator.next(ReduceContext.java:163)

Am I doing something wrong?  Should I expect this VALUEIN object to  
change from underneath me?  I'm using hadoop 0.20.1 (from a cloudera  
tarball)

Chris