How can I determine ascending or descending of the method Collections.sort

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.

package lambdaExpression;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        List<String> names = Arrays.asList("peter", "anna", "mike", "xenia", "mike");

        System.out.println("names before sort are:");
        for (String x : names) {
            System.out.println(x);
        }

        // Method 1 , Ascending
        Collections.sort(names, (String a, String b) -> {
            return a.compareTo(b);
        });

        System.out.println("names After sort1 are: ");
        myout((String[]) names.toArray());

        // Method 2 , Descending
        Collections.sort(names, (String a, String b) -> b.compareTo(a));

        System.out.println("names After sort2 are: ");
        myout((String[]) names.toArray());

        // Method 3 , Ascending
        Collections.sort(names, (a, b) -> a.compareTo(b));

        System.out.println("names After sort3 are: ");
        myout((String[]) names.toArray());

        // Method 4 , No swapping,keep prior change result
        Collections.sort(names, (String a, String b) -> {
            return a.compareTo(a);
        });

        System.out.println("names After sort4 are: ");
        myout((String[]) names.toArray());

    }

    public static void myout(String[] names) {
        for (String x : names) {
            System.out.println(x);
        }
    }

}