Collections类可以将存储与List中的元素进行排序，可以按照针对元素的排序方法进行排序，也可以按照指定的排序类进行排序。
Collections类提供了两个静态的sort方法：
sort(List<T> list)
sort(List<T> list, Comparator<? super T> c)
第一个方法是直接将List中的元素进行排序，排序方法需要List中存储的元素来提供，即存储的元素要是可排序的；
第二个方法除了提供要排序的List外，还需要提供一个指定的排序类，来指定排序规则，该List中存储的元素可不实现可排序接口。
从源代码查看它们具体的实现：
public static <T extends Comparable<? super T>> void sort(List<T> list) {
Object[] a = list.toArray();
Arrays.sort(a);
ListIterator<T> i = list.listIterator();
for (int j=0; j<a.length; j++) {
i.next();
i.set((T)a[j]);
}
}
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]);
}
}Collections的两个sort方法分别调用了Arrays的两个对应的sort方法，将排序的实现推给了Arrays去实现，下面我们看看Arrays中是怎么实现的sort方法：
public static void sort(Object[] a) {
Object[] aux = (Object[])a.clone();
mergeSort(aux, a, 0, a.length, 0);
}
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++];
}
}不带比较器参数的Arrays.sort方法调用了不带比较器的mergeSort方法，在mergeSort的实现中调用了存储对象本身的compareTo方法，因此利用该方法进行sort排序的对象必须实现compareTo方法。
public static <T> void sort(T[] a, Comparator<? super T> c) {
T[] aux = (T[])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,
Comparator c) {
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 && c.compare(dest[j-1], 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, c);
mergeSort(dest, src, mid, high, -off, c);
// 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 (c.compare(src[mid-1], 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 && c.compare(src[p], src[q]) <= 0)
dest[i] = src[p++];
else
dest[i] = src[q++];
}
}带比较器参数的Arrays.sort方法调用了带比较器的mergeSort方法，在mergeSort的实现中调用了比较器的compareTo方法，而没有调用存储对象本身的。只需指定一个比较器即可。
