Arrays.sort和Collections.sort实现原理解析_软件开发

超、凢脫俗 2023-02-25 02:17 1阅读 0赞

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

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

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDc4MjE0MA_size_16_color_FFFFFF_t_70][]

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;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDc4MjE0MA_size_16_color_FFFFFF_t_70 1][]

//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;

[叩丁狼][Link 1]Arrays.sort和Collections.sort实现原理解析

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

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDc4MjE0MA_size_16_color_FFFFFF_t_70 2][]

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

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDc4MjE0MA_size_16_color_FFFFFF_t_70 3][]

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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDc4MjE0MA_size_16_color_FFFFFF_t_70 2]: /images/20230209/4ff372fec3154b26b4b714e9f6ed336f.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDc4MjE0MA_size_16_color_FFFFFF_t_70 3]: /images/20230209/cf69d7eb906c4a56b31a3e182178ecd3.png