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Arrays.sort和Collections.sort实现原理解析

创建时间:2019年04月17日16时25分 来源:传智播客java培训

    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 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

Arrays.sort和Collections.sort实现原理解析

  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)

arrays.sort

  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

Arrays.sort和Collections.sort实现原理解析

  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 void sort(T[] a, int lo, int hi, Comparator 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 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(文章转载于csdn.net/u011410529/article/details/56668545