Mapreduce实例-分组排重（group by distinct）

public class GroupComparator implements RawComparator<MyBinaryKey> {
 
 @Override
 public int compare(MyBinaryKey o1, MyBinaryKey o2) {
  return o1.toString().compareTo(o2.toString());
 }

 @Override
 public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
  return WritableComparator.compareBytes(b1, s1, Long.SIZE / 8 + Integer.SIZE / 8 * 3, b2, s2,  Long.SIZE / 8 + Integer.SIZE / 8 * 3);
 }

}

public abstract class UVBinaryKey  extends BinaryComparable implements WritableComparable<BinaryComparable>{
 //根据需要添加属性；
  @Override
 public void readFields(DataInput in) throws IOException {

} 

@Override
 public byte[] getBytes() {

}

}

public class MyPartitioner extends Partitioner<MyBinaryKey, NullWritable> {

 /**
  * 根据uv/ip取模分区，保证相同uv/ip落在同一分区
  */
 @Override
 public int getPartition(MyBinaryKey key, NullWritable value, int numPartitions) {
  
  int k=0;
  for(byte b : key.getAttr()){
   k+=b&0xff;
  }
  return k%numPartitions;
 }

}



  job.setMapOutputKeyClass(UVBinaryKey.class);
  job.setGroupingComparatorClass(GroupComparator.class);
   job.setPartitionerClass(MyPartitioner.class);

  map 略

combiner（根据需要添加）
reduce中的实现：
       @Override
        protected void reduce(UVBinaryKey key, Iterable<NullWritable> values, Context context)
                throws IOException,
                InterruptedException {
            long count = 0;
            byte[] tbsign = null;
            for (NullWritable nullWritable : values) {
                byte[] attr = key.getAttr();
                if (tbsign == null) {
                    tbsign = attr;
                    count++;
                }
                if (tbsign != null) {
                    if (tbsign.length != attr.length) {
                        count++;
                        tbsign = attr;
                    } else {
                        for (int i = 0; i < tbsign.length; i++) {
                            if (tbsign[i] != attr[i]) {
                                count++;
                                tbsign = attr;
                                break;
                            }
                        }
                    }
                }

            }
            StringBuffer out = new StringBuffer();
            out.append(new String(key.getCity()))
                    .append(Constants.FIELDS_TERMINATED).append(count);
            context.write(new Text(out.toString()), NullWritable.get());

        }