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]