hadoop中compare函数

在看hadoop  的二次排序的时候，改写了下， 加了第三个参数,  本来以为是在

   public int compareTo(IntPair o) {
      System.out.println("-----------compareTo");
      if (first != o.first) {
        return first < o.first ? -1 : 1;
      } else if (second != o.second) {
        return second < o.second ? -1 : 1;
      }else if (third != o.third) {
    	        return third < o.third ? -1 : 1;}

        return 0;
      }

本来以为排序在这里面进行， 后来发现不是，把比较第3个字段的代码去掉， 发现还是有序的。

后来通过打印得知在compare函数中，稍微改写了下

      public int compare(byte[] b1, int s1, int l1,
                         byte[] b2, int s2, int l2) {
       // 二进制数组读取
       int intvalue = readInt(b1, s1);
       System.out.println("s1 = " +  b1.length);
       　　　　// 验证b1中存储的数据
       int third = 0;
       for(int i =s1 + 9; i<= s1+ 12; i++){
           third += (b1[i]&0xff) << (24-8*i);
        }
           System.out.println("third = " + third);

        return compareBytes(b1, s1, l1, b2, s2, l2);
      }
    }

有3个整形值， s1为开始位置， l1为长度12， 这样我们就可以读出我们的值

return compareBytes(b1, s1, l1, b2, s2, l2);调用 return FastByteComparisons.compareTo(b1, s1, l1, b2, s2, l2);

    public int compareTo(byte[] buffer1, int offset1, int length1, byte[] buffer2, int offset2, int length2)
      {
        if ((buffer1 == buffer2) && (offset1 == offset2) && (length1 == length2))
        {
          return 0;
        }

        int end1 = offset1 + length1;
        int end2 = offset2 + length2;
        int i = offset1; for (int j = offset2; (i < end1) && (j < end2); ++j) {
          int a = buffer1[i] & 0xFF;
          int b = buffer2[j] & 0xFF;
          if (a != b)
            return (a - b);
          ++i;
        }

        return (length1 - length2);
      }
    }
  }

从代码中就知道了，通过字节数组比较三个值， 这样就出来的结果就是有序的了

结论， 理论上N个字段这样出来的结果的都是有序的，只是比较的长度有所变化

测试又加了一个字段， 输出结果都是有序的。

测试代码

  public static class IntPair
                      implements WritableComparable<IntPair> {
    private int first = 0;
    private int second = 0;
    private int third = 0;
    private int fourth = 0;

    /**
     * Set the left and right values.
     */
    public void set(int left, int right, int third, int fourth) {
      first = left;
      second = right;
      this.third = third;
      this.fourth = fourth;
    }

    public int getFirst() {
      return first;
    }

    public int getSecond() {
      return second;
    }

    public int getThird() {
        return third;
      }

    public int getFourth() {
        return fourth;
      }

    @Override
	public String toString() {
    	System.out.println("third = " + third);
    	return first + "\t" + second + "\t" + third + "\t" + fourth;
	}

	/**
     * 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;
      third = in.readInt();// + Integer.MIN_VALUE;
      fourth = in.readInt();
    }
    @Override
    public void write(DataOutput out) throws IOException {
     /*
      out.writeInt(first - Integer.MIN_VALUE);
      out.writeInt(second - Integer.MIN_VALUE);
      out.writeInt(third - Integer.MIN_VALUE);
      */
        out.writeInt(first );
        out.writeInt(second );
        out.writeInt(third );
        out.writeInt(fourth);
    }
    @Override
    public int hashCode() {
      return first * 157 + second*10 + third;
    }

    @Override
    public boolean equals(Object right) {
      if (right instanceof IntPair) {
        IntPair r = (IntPair) right;
        return r.first == first && r.second == second && r.third == third && r.fourth == fourth;
      } else {
        return false;
      }
    }

    /** A Comparator that compares serialized IntPair. */
    public static class Comparator extends WritableComparator {
      public Comparator() {
        super(IntPair.class);
      }

      // 排序比较器，数据全部存在byte数组
      public int compare(byte[] b1, int s1, int l1,
                         byte[] b2, int s2, int l2) {
       // 二进制数组读取
       int intvalue = readInt(b1, s1);
       System.out.println("s1 = " +  b1.length);

       int third = 0;
       for(int i =s1 + 9; i<= s1+ 12; i++){
    	   third += (b1[i]&0xff) << (24-8*i);
    	}
       	System.out.println("third = " + third);

        return compareBytes(b1, s1, l1, b2, s2, l2);
      }
    }

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

    // 好像没用上
    @Override
    public int compareTo(IntPair o) {
      System.out.println("-----------compareTo");
      if (first != o.first) {
        return first < o.first ? -1 : 1;
      } else if (second != o.second) {
        return second < o.second ? -1 : 1;
      }// else if (third != o.third) {
    	//        return third < o.third ? -1 : 1;}

        return 0;
      }
  }

hadoop中compare函数