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]
