java.util.Arrays.sort方法简单分析

今天比较仔细的观察了下1.6jdk下的java.util.Arrays.sort方法

稍微总结下：

sort分为对基本数据类型(byte char short int float long double)和object类型的排序

1.对基本数据类型的排序

Arrays类中public方法都是sort，

但里面实际做工作的是sort2和sort1

最实际的工作是sort1

sort2是做一些边界处理的闲杂工作，类似于

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'ssort1(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; } }

几个核心的sort1方法声明：

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)

实现逻辑基本相同，如果数组的长度小于7，则是最简单的插入排序，否则采用快速排序

由于快速排序在最坏情况下的时间复杂度为O(n^2)，故标志元素的选取也是有讲究的，

类似于

if (len > 7) { int l = off; int n = off + len - 1; if (len > 40) { // Big arrays, pseudomedian of 9int 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.对object数据类型的排序，主要是采用归并排序

主要实现方法

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)

这两个，

在归并过程中如果数组长度小于7，则采用插入排序使数组有序，

否则继续递归归并，完成排序。