定义IntPair 以及 IntPair（first,second）的compareto，先比較first的大小，再比較second的大小

定义FirstPartitioner是为了让partition的时候依照IntPair的first来做为选择reduce的根据

定义FirstGroupingComparator是为了：《Pro Hadoop》，里面有一部分内容详解了这个问题，看后最终明确了，和大家分享一下。reduce方法每次是读一条记录，读到对应的key，可是处理value集合时，处理完当前记录的values后，还会推断下一条记录是不是和当前的key是不是同一个组，假设是的话，会继续读取这些记录的值，而这个记录也会被觉得已经处理了，直到记录不是当前组，这次reduce调用才结束，这样一次reduce调用就会处理掉一个组中的全部记录，而不不过一条了。

以下是从hadoop里取出的源码，能够再理解下：

/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
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package org.apache.hadoop.examples;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.util.StringTokenizer;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.RawComparator;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Partitioner;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.util.GenericOptionsParser;

/**
 * This is an example Hadoop Map/Reduce application.
 * It reads the text input files that must contain two integers per a line.
 * The output is sorted by the first and second number and grouped on the 
 * first number.
 *
 * To run: bin/hadoop jar build/hadoop-examples.jar secondarysort
 *            <i>in-dir</i> <i>out-dir</i> 
 */
public class SecondarySort {
 
  /**
   * Define a pair of integers that are writable.
   * They are serialized in a byte comparable format.
   */
  public static class IntPair 
                      implements WritableComparable<IntPair> {
    private int first = 0;
    private int second = 0;
    
    /**
     * Set the left and right values.
     */
    public void set(int left, int right) {
      first = left;
      second = right;
    }
    public IntPair(){}
    public IntPair(int left,int right){
    	set(left, right);
    }
    public int getFirst() {
      return first;
    }
    public int getSecond() {
      return second;
    }
    /**
     * Read the two integers. 
     * Encoded as: MIN_VALUE -> 0, 0 -> -MIN_VALUE, MAX_VALUE-> -1
     */
    @Override
    public void readFields(DataInput in) throws IOException {
      first = in.readInt() + Integer.MIN_VALUE;
      second = in.readInt() + Integer.MIN_VALUE;
    }
    @Override
    public void write(DataOutput out) throws IOException {
      out.writeInt(first - Integer.MIN_VALUE);
      out.writeInt(second - Integer.MIN_VALUE);
    }
    @Override
    public int hashCode() {
      return first * 157 + second;
    }
    @Override
    public boolean equals(Object right) {
      if (right instanceof IntPair) {
        IntPair r = (IntPair) right;
        return r.first == first && r.second == second;
      } else {
        return false;
      }
    }
    /** A Comparator that compares serialized IntPair. */ 
    public static class Comparator extends WritableComparator {
      public Comparator() {
        super(IntPair.class);
      }

      public int compare(byte[] b1, int s1, int l1,
                         byte[] b2, int s2, int l2) {
        return compareBytes(b1, s1, l1, b2, s2, l2);
      }
    }

    static {                                        // register this comparator
      WritableComparator.define(IntPair.class, new Comparator());
    }

    @Override
    public int compareTo(IntPair o) {
      if (first != o.first) {
        return first < o.first ? -1 : 1;
      } else if (second != o.second) {
        return second < o.second ? -1 : 1;
      } else {
        return 0;
      }
    }
  }
  
  /**
   * Partition based on the first part of the pair.
   */
  public static class FirstPartitioner extends Partitioner<IntPair,IntWritable>{
    @Override
    public int getPartition(IntPair key, IntWritable value, 
                            int numPartitions) {
      return Math.abs(key.getFirst() * 127) % numPartitions;
    }
  }

  /**
   * Compare only the first part of the pair, so that reduce is called once
   * for each value of the first part.
   */
  public static class FirstGroupingComparator 
                implements RawComparator<IntPair> {
    @Override
    public int compare(byte[] b1, int s1, int l1, byte[] b2, int s2, int l2) {
      return WritableComparator.compareBytes(b1, s1, Integer.SIZE/8, 
                                             b2, s2, Integer.SIZE/8);
    }

    @Override
    public int compare(IntPair o1, IntPair o2) {
      int l = o1.getFirst();
      int r = o2.getFirst();
      return l == r ? 0 : (l < r ? -1 : 1);
    }
  }

  /**
   * Read two integers from each line and generate a key, value pair
   * as ((left, right), right).
   */
  public static class MapClass 
         extends Mapper<LongWritable, Text, IntPair, IntWritable> {
    
    private final IntPair key = new IntPair();
    private final IntWritable value = http://www.mamicode.com/new IntWritable();>