Gautam, You are right. The treeset is treated like any collection object within MR   The following Mapper code worked for me   import java.io.IOException; import java.util.Iterator; import java.util.TreeSet; import org.apache.hadoop.io.LongWritable; import org.apache.hadoop.io.NullWritable; import org.apache.hadoop.io.Text; import org.apache.hadoop.mapreduce.Mapper; public class KPWordCountMapper extends Mapper<LongWritable, Text, Text, NullWritable>{ int count = 0; @Override public void map(LongWritable inputKey,Text inputVal,Context context) throws IOException,InterruptedException { TreeSet<String> ts = new TreeSet<>(); String line = inputVal.toString(); String[] splits = line.split("\\W+"); for(String outputKey:splits) if(outputKey.length() > 0){ ts.add(outputKey); } Iterator<String> itr= ts.iterator(); while(itr.hasNext()){ context.write(new Text(itr.next()),NullWritable.get()); } } } ... View more