It depends on the Int number the Comparator returns.
If return <0, when compare each two element of the collection, if a>b, swap their position.
If return =0, no need position swapping.
If return >0, when compare each two element of the collection, if a<b, swap their position.
See the below demo code to help you understand this.
1 package lambdaExpression;
2
3 import java.util.Arrays;
4 import java.util.Collections;
5 import java.util.List;
6
7 public class Main {
8
9 public static void main(String[] args) {
10 // TODO Auto-generated method stub
11
12 List<String> names = Arrays.asList("peter", "anna", "mike", "xenia", "mike");
13
14 System.out.println("names before sort are:");
15 for (String x : names) {
16 System.out.println(x);
17 }
18
19 // Method 1 , Ascending
20 Collections.sort(names, (String a, String b) -> {
21 return a.compareTo(b);
22 });
23
24 System.out.println("names After sort1 are: ");
25 myout((String[]) names.toArray());
26
27 // Method 2 , Descending
28 Collections.sort(names, (String a, String b) -> b.compareTo(a));
29
30 System.out.println("names After sort2 are: ");
31 myout((String[]) names.toArray());
32
33 // Method 3 , Ascending
34 Collections.sort(names, (a, b) -> a.compareTo(b));
35
36 System.out.println("names After sort3 are: ");
37 myout((String[]) names.toArray());
38
39 // Method 4 , No swapping,keep prior change result
40 Collections.sort(names, (String a, String b) -> {
41 return a.compareTo(a);
42 });
43
44 System.out.println("names After sort4 are: ");
45 myout((String[]) names.toArray());
46
47 }
48
49 public static void myout(String[] names) {
50 for (String x : names) {
51 System.out.println(x);
52 }
53 }
54
55 }
