Collections sort() 合并排序

Collections 的sort方法使用的是合并排序算法，使用时注意list中的元素需要实现Comparable接口。

 这里有个简单的例子：

  1 public int compareTo(Object obj) {

 2         if (obj instanceof TestObject) {
 3             TestObject to = (TestObject) obj;
 4             if (this.getLength() > to.getLength()) {
 5                 return 1;
 6             } else if (this.getLength() == to.getLength()) {
 7                 return 0;
 8             } else {
 9                 return -1;
10             }
11         } else {
12             throw new ClassCastException();
13         }
14     }

下面是源码： 

 1     /**
 2      * Sorts the specified list into ascending order, according to the
 3      * <i>natural ordering</i> of its elements.  All elements in the list must
 4      * implement the <tt>Comparable</tt> interface.  Furthermore, all elements
 5      * in the list must be <i>mutually comparable</i> (that is,
 6      * <tt>e1.compareTo(e2)</tt> must not throw a <tt>ClassCastException</tt>
 7      * for any elements <tt>e1</tt> and <tt>e2</tt> in the list).<p>
 8      *
 9      * This sort is guaranteed to be <i>stable</i>:  equal elements will
10      * not be reordered as a result of the sort.<p>
11      *
12      * The specified list must be modifiable, but need not be resizable.<p>
13      *
14      * The sorting algorithm is a modified mergesort (in which the merge is
15      * omitted if the highest element in the low sublist is less than the
16      * lowest element in the high sublist).  This algorithm offers guaranteed
17      * n log(n) performance.
18      *
19      * This implementation dumps the specified list into an array, sorts
20      * the array, and iterates over the list resetting each element
21      * from the corresponding position in the array.  This avoids the
22      * n<sup>2</sup> log(n) performance that would result from attempting
23      * to sort a linked list in place.
24      *
25      * @param  list the list to be sorted.
26      * @throws ClassCastException if the list contains elements that are not
27      *           <i>mutually comparable</i> (for example, strings and integers).
28      * @throws UnsupportedOperationException if the specified list's
29      *           list-iterator does not support the <tt>set</tt> operation.
30      * @see Comparable
31      */
32     public static <T extends Comparable<? super T>> void sort(List<T> list) {
33     Object[] a = list.toArray();
34     Arrays.sort(a);
35     ListIterator<T> i = list.listIterator();
36     for (int j=0; j<a.length; j++) {
37         i.next();
38         i.set((T)a[j]);
39     }
40     }