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Java Arrays.sort和Collections.sort Java Arrays.sort和Collections.sort排序实现原理解析

一年e度的夏天 人气:0

1、使用

排序

2、原理

事实上Collections.sort方法底层就是调用的array.sort方法,而且不论是Collections.sort或者是Arrays.sort方法,

跟踪下源代码吧,首先我们写个demo

public static void main(String[] args) {
         List<String> strings = Arrays.asList("6", "1", "3", "1","2");

         Collections.sort(strings);//sort方法在这里

         for (String string : strings) {

           System.out.println(string);
         }
       }

简单得不能再简单的方法了,让我们一步步跟踪

OK,往下面看,发现collections.sort方法调用的list.sort

然后跟踪一下,list里面有个sort方法,但是list是一个接口,肯定是调用子类里面的实现,这里我们demo使用的是一个Arrays.asList方法,所以事实上我们的子类就是arraylist了。OK,看arraylist里面sort实现,选择第一个,为什么不选择第二个呢?(可以看二楼评论,解答得很正确,简单说就是用Arrays.sort创建的ArrayList对象)

OK,发现里面调用的Arrays.sort(a, c); a是list,c是一个比较器,我们来看一下这个方法

我们没有写比较器,所以用的第二项,LegacyMergeSort.userRequested这个bool值是什么呢?
跟踪这个值,我们发现有这样的一段定义:

> Old merge sort implementation can be selected (for
> compatibility with broken comparators) using a system property.
> Cannot be a static boolean in the enclosing class due to
> circular dependencies. To be removed in a future release.

反正是一种老的归并排序,不用管了现在默认是关的

OK,我们走的是sort(a)这个方法,接着进入这个

接着看我们重要的sort方法

static void sort(Object[] a, int lo, int hi, Object[] work, int workBase, int workLen) {
         assert a != null && lo >= 0 && lo <= hi && hi <= a.length;

         int nRemaining = hi - lo;
         if (nRemaining < 2)
           return; // array的大小为0或者1就不用排了

         // 当数组大小小于MIN_MERGE(32)的时候,就用一个"mini-TimSort"的方法排序,jdk1.7新加
         if (nRemaining < MIN_MERGE) {
          //这个方法比较有意思,其实就是将我们最长的递减序列,找出来,然后倒过来
           int initRunLen = countRunAndMakeAscending(a, lo, hi);
           //长度小于32的时候,是使用binarySort的
           binarySort(a, lo, hi, lo + initRunLen);
           return;
         }
        //先扫描一次array,找到已经排好的序列,然后再用刚才的mini-TimSort,然后合并,这就是TimSort的核心思想
         ComparableTimSort ts = new ComparableTimSort(a, work, workBase, workLen);
         int minRun = minRunLength(nRemaining);
         do {
           // Identify next run
           int runLen = countRunAndMakeAscending(a, lo, hi);

           // If run is short, extend to min(minRun, nRemaining)
           if (runLen < minRun) {
             int force = nRemaining <= minRun ? nRemaining : minRun;
             binarySort(a, lo, lo + force, lo + runLen);
             runLen = force;
           }

           // Push run onto pending-run stack, and maybe merge
           ts.pushRun(lo, runLen);
           ts.mergeCollapse();

           // Advance to find next run
           lo += runLen;
           nRemaining -= runLen;
         } while (nRemaining != 0);

         // Merge all remaining runs to complete sort
         assert lo == hi;
         ts.mergeForceCollapse();
         assert ts.stackSize == 1;
     }

回到5,我们可以看到当我们写了比较器的时候就调用了TimSort.sort方法,源码如下

static <T> void sort(T[] a, int lo, int hi, Comparator<? super T> c,
                 T[] work, int workBase, int workLen) {
         assert c != null && a != null && lo >= 0 && lo <= hi && hi <= a.length;

         int nRemaining = hi - lo;
         if (nRemaining < 2)
           return; // Arrays of size 0 and 1 are always sorted

         // If array is small, do a "mini-TimSort" with no merges
         if (nRemaining < MIN_MERGE) {
           int initRunLen = countRunAndMakeAscending(a, lo, hi, c);
           binarySort(a, lo, hi, lo + initRunLen, c);
           return;
         }

         /**
         * March over the array once, left to right, finding natural runs,
         * extending short natural runs to minRun elements, and merging runs
         * to maintain stack invariant.
         */
      TimSort<T> ts = new TimSort<>(a, c, work, workBase, workLen);
        int minRun = minRunLength(nRemaining);
        do {
          // Identify next run
          int runLen = countRunAndMakeAscending(a, lo, hi, c);

          // If run is short, extend to min(minRun, nRemaining)
          if (runLen < minRun) {
            int force = nRemaining <= minRun ? nRemaining : minRun;
            binarySort(a, lo, lo + force, lo + runLen, c);
            runLen = force;
          }

          // Push run onto pending-run stack, and maybe merge
          ts.pushRun(lo, runLen);
          ts.mergeCollapse();

          // Advance to find next run
          lo += runLen;
          nRemaining -= runLen;
        } while (nRemaining != 0);

        // Merge all remaining runs to complete sort
        assert lo == hi;
        ts.mergeForceCollapse();
        assert ts.stackSize == 1;
   }

和上面的sort方法是一样的,其实也就是TimSort的源代码

3、总结

不论是Collections.sort方法或者是Arrays.sort方法,底层实现都是TimSort实现的,这是jdk1.7新增的,以前是归并排序。TimSort算法就是找到已经排好序数据的子序列,然后对剩余部分排序,然后合并起来

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