DummyOutputCollector<StringTuple,DoubleWritable> reducerOutput = new DummyOutputCollector<StringTuple,DoubleWritable>();
Map<StringTuple, List<DoubleWritable>> outputData = mapperOutput.getData();
// put the mapper output in the expected order (emulate shuffle)
FeatureLabelComparator cmp = new FeatureLabelComparator();
Collection<StringTuple> keySet = new TreeSet<StringTuple>(cmp);
keySet.addAll(mapperOutput.getKeys());
for (StringTuple k: keySet) {
List<DoubleWritable> v = outputData.get(k);