How to use Collections.reverse in java to sort list collections in descending order

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Use Collections.reverse to sort the list collection in descending order

Today, I inadvertently searched Collections.reverse this method, and found that some people have a deep misunderstanding about it. Reverse can sort a specified list in descending order, but its output is not in descending order.

Indeed, it is possible to use Collections.reverse in combination with certain methods to sort list collections in descending order, but it is wrong to directly use Collections.reverse (list) to sort descending.

Reverse means reverse, not descending. It just reverses the original order of the list collection, which does not mean that the order is descending. So if you want to achieve descending order, you can first ascend the collection, and then reverse it, so it is descending.

For example:

Import java.util.*;public class Test {private static Map map = new HashMap (); public static void main (String [] args) {long [] data = {1506326821000l, 1506327060000l, 1506326880000l, 15063270000l, 1506326940000l, 1506326760000l, 150632626700l}; List list = new ArrayList (); for (long key: data) {list.add (key);} System.out.println (list); / / ascending Collections.sort (list); System.out.println (list) / / reverse Collections.reverse (list); System.out.println (list);}}

Output:

[1506326821000, 1506327060000, 1506326880000, 1506327000000, 1506326940000, 1506326760000, 1506326700000]

[1506326700000, 1506326760000, 1506326821000, 1506326880000, 1506326940000, 1506327000000, 1506327060000]

[1506327060000, 1506327000000, 1506326940000, 1506326880000, 1506326821000, 1506326760000, 1506326700000]

Collections.reverse principle public static void reverse (List list) {int size = list.size (); / / REVERSE_THRESHOLD = 18 data flashback size limit if (size

< REVERSE_THRESHOLD || list instanceof RandomAccess) { //折半处理，前后交换 //size>

> 1 equals size/2 for (int item0, mid=size > > 1, jambisizemai 1; I > 1; I