java.util.Arrays.sort方法簡單分析

今天比較仔細的觀察了下1.6jdk下的java.util.Arrays.sort方法

稍微總結下：

sort分爲對基本數據類型(byte char short int float long double)和object類型的排序

1.對基本數據類型的排序

Arrays類中public方法都是sort，

但裏面實際做工作的是sort2和sort1

最實際的工作是sort1

sort2是做一些邊界處理的閒雜工作，類似於

private static void sort2(double a[], int fromIndex, int toIndex) { final long NEG_ZERO_BITS = Double.doubleToLongBits(-0.0d); /* * The sort is done in three phases to avoid the expense of using * NaN and -0.0 aware comparisons during the main sort. */ /* * Preprocessing phase: Move any NaN's to end of array, count the * number of -0.0's, and turn them into 0.0's. */ int numNegZeros = 0; int i = fromIndex, n = toIndex; while(i < n) { if (a[i] != a[i]) { double swap = a[i]; a[i] = a[--n]; a[n] = swap; } else { if (a[i]==0 && Double.doubleToLongBits(a[i])==NEG_ZERO_BITS) { a[i] = 0.0d; numNegZeros++; } i++; } } // Main sort phase: quicksort everything but the NaN's sort1(a, fromIndex, n-fromIndex); // Postprocessing phase: change 0.0's to -0.0's as required if (numNegZeros != 0) { int j = binarySearch0(a, fromIndex, n, 0.0d); // posn of ANY zero do { j--; } while (j>=0 && a[j]==0.0d); // j is now one less than the index of the FIRST zero for (int k=0; k<numNegZeros; k++) a[++j] = -0.0d; } }

幾個核心的sort1方法聲明：

private static void sort1(long x[], int off, int len)

private static void sort1(int x[], int off, int len)

private static void sort1(short x[], int off, int len)

private static void sort1(char x[], int off, int len)

private static void sort1(byte x[], int off, int len)

private static void sort1(double x[], int off, int len)

private static void sort1(float x[], int off, int len)

實現邏輯基本相同，如果數組的長度小於7，則是最簡單的插入排序，否則採用快速排序

由於快速排序在最壞情況下的時間複雜度爲O(n^2)，故標誌元素的選取也是有講究的，

類似於

if (len > 7) { int l = off; int n = off + len - 1; if (len > 40) { // Big arrays, pseudomedian of 9 int s = len/8; l = med3(x, l, l+s, l+2*s); m = med3(x, m-s, m, m+s); n = med3(x, n-2*s, n-s, n); } m = med3(x, l, m, n); // Mid-size, med of 3 }

中的big arrays部分

2.對object數據類型的排序，主要是採用歸併排序

主要實現方法

private static void mergeSort(Object[] src,
      Object[] dest,
      int low,
      int high,
      int off)

以及

 private static void mergeSort(Object[] src,
      Object[] dest,
      int low, int high, int off,
      Comparator c)

這兩個，

在歸併過程中如果數組長度小於7，則採用插入排序使數組有序，

否則繼續遞歸歸併，完成排序。