http://121dog198.blog.163.com/blog/static/50859950201431661150523/

Java中如果需要对一个collections排序，需要继承于Comparable或者comparator接口，那么使用的排序算法是什么呢，一般情况下，排序算法包括：插入排序、快速排序、合并排序、冒泡排序等，java的Collections.sort算法调用的是合并排序，它是稳定排序，当数据接近有序的时候，效率更高，collections中的数据在排序前需要输入到array中，接着调用Arrays.sort函数来完成对象排序，最近通过迭代器将数组中排好序的对象些人到collection中，这也要求collection必须为mutable类型的。合并排序的大致过程为：
void mergerSort(int[] a){
 int len = a.lenght()
 int mid = len>>2
 if(len>1){
   int[] pre=a[0:mid);
   int[] after=a[mid:len);
   mergerSort(pre);
   mergerSort(after);
  merge(a,pre,after)
}
}
1.collections转化为array，并借助于arrays的sort功能完成排序，并回写到collection
 public static <T> void sort(List<T> list, Comparator<? super T> c) {
        Object[] a = list.toArray();
        Arrays.sort(a, (Comparator)c);
        ListIterator i = list.listIterator();
        for (int j=0; j<a.length; j++) {
            i.next();
            i.set(a[j]);
        }
    }
2. Arrays合并排序的实现：
public static <T> void sort(T[] a, Comparator<? super T> c) {
        if (LegacyMergeSort.userRequested)
            legacyMergeSort(a, c);
        else
            TimSort.sort(a, c);
    }

    /** To be removed in a future release. */
    private static <T> void legacyMergeSort(T[] a, Comparator<? super T> c) {
        T[] aux = a.clone();
        if (c==null)
            mergeSort(aux, a, 0, a.length, 0);
        else
            mergeSort(aux, a, 0, a.length, 0, c);
    }
private static void mergeSort(Object[] src,
                                  Object[] dest,
                                  int low,
                                  int high,
                                  int off) {
        int length = high - low;

        // Insertion sort on smallest arrays
        if (length < INSERTIONSORT_THRESHOLD) {
            for (int i=low; i<high; i++)
                for (int j=i; j>low &&
                         ((Comparable) dest[j-1]).compareTo(dest[j])>0; j--)
                    swap(dest, j, j-1);
            return;
        }

        // Recursively sort halves of dest into src
        int destLow  = low;
        int destHigh = high;
        low  += off;
        high += off;
        int mid = (low + high) >>> 1;
        mergeSort(dest, src, low, mid, -off);
        mergeSort(dest, src, mid, high, -off);

        // If list is already sorted, just copy from src to dest.  This is an
        // optimization that results in faster sorts for nearly ordered lists.
        if (((Comparable)src[mid-1]).compareTo(src[mid]) <= 0) {
            System.arraycopy(src, low, dest, destLow, length);
            return;
        }

        // Merge sorted halves (now in src) into dest
        for(int i = destLow, p = low, q = mid; i < destHigh; i++) {
            if (q >= high || p < mid && ((Comparable)src[p]).compareTo(src[q])<=0)
                dest[i] = src[p++];
            else
                dest[i] = src[q++];
        }
    }



注>>:二进制右移，左侧补符号位，>>>:二进制右移，左侧补无符号为，也就是0

3.举例：
public class TestCompare {

    private String com;
    private int id;

    public TestCompare(int id, String com) {
        super();
        this.com = com;
        this.id = id;
    }

    @Override
    public String toString() {
        return "TestCompare [com=" + com + ", id=" + id + "]";
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub

        List<TestCompare> li = new ArrayList<TestCompare>();
        li.add(new TestCompare(1, null));
        li.add(new TestCompare(2, "dfsd"));
        li.add(new TestCompare(3, null));
        li.add(new TestCompare(4, "ying"));

        Collections.sort(li, new Comparator<TestCompare>() {

            @Override
            public int compare(TestCompare o1, TestCompare o2) {
                // TODO Auto-generated method stub
                if (o1.com == o2.com)
                    return 0;
                else if (o1.com == null)
                    return 1;
                else if (o2.com == null)
                    return -1;
                else
                    return o1.com.compareTo(o2.com);
            }

        });

List中含有4个元素，根据合并排序的算法，首先分为[0:2) 和[2:4)
接着[0,2)分为[0:1) 和[1:2)
[0:1):TestCompare [com=null, id=1]
[1:2):TestCompare [com=dfsd, id=2]
合并排序后为
TestCompare [com=dfsd, id=2]
TestCompare [com=null, id=1]
接着执行[2:4),分为[2:3) 和[3:4)
[2:3):TestCompare [com=null, id=3]
[3:4):TestCompare [com=ying, id=4]
合并排序后为：
TestCompare [com=ying, id=4]
TestCompare [com=null, id=3]

将两组合并的数据进行再次合并，及为：
TestCompare [com=dfsd, id=2]
TestCompare [com=ying, id=4]
TestCompare [com=null, id=1]
TestCompare [com=null, id=3]

补充：Arrays.sort() 采用了2种排序算法 — 基本类型数据使用快速排序法，对象数组使用归并排序。