hadoop中compare函數

標籤：style   blog   java   color   os   資料   

在看hadoop  的二次排序的時候，改寫了下， 加了第三個參數,  本來以為是在

 

   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;      }

 本來以為排序在這裡面進行， 後來發現不是，把比較第3個欄位的代碼去掉， 發現還是有序的。

後來通過列印得知在compare函數中，稍微改寫了下

      public int compare(byte[] b1, int s1, int l1,                         byte[] b2, int s2, int l2) {        // 位元組讀取       int intvalue = readInt(b1, s1);           System.out.println("s1 = " +  b1.length);       
　　　　// 驗證b1中儲存的資料       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個整形值， s1為開始位置， l1為長度12， 這樣我們就可以讀出我們的值

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

 從代碼中就知道了，通過位元組數組比較三個值， 這樣就出來的結果就是有序的了

結論， 理論上N個欄位這樣出來的結果的都是有序的，只是比較的長度有所變化

測試又加了一個欄位， 輸出結果都是有序的。

測試代碼

  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;      }        @Overridepublic 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);      }            // 排序比較子，資料全部存在byte數組      public int compare(byte[] b1, int s1, int l1,                         byte[] b2, int s2, int l2) {        // 位元組讀取       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());    }    // 好像沒用上        @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;      }  }