Detailed explanation of JAVA using Comparator interface to realize custom sorting

1. Principles

The Comparator interface enables custom sorting, and when implementing the Comparator interface, override the compare method:
int compare (Object o1, Object o2) Returns an integer of 1 basic type
If you want to sort in ascending order, o1 is less than o2 and returns-1 (negative number), equal returns 0, and 01 is greater than 02 and returns 1 (positive number)
If you want to sort in descending order, o1 is less than o2 and returns 1 (positive number), equal returns 0, and 01 is greater than 02 and returns-1 (negative number)

 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.List;
 
 public class UserComparator implements Comparator < User > {
 
 public static void main( String[] args ) {
 List < User > users = new ArrayList < User >();
 users.add( new User( 10, "a" ) );
 users.add( new User( 11, "d" ) );
 users.add( new User( 15, "s" ) );
 users.add( new User( 6, "x" ) );
 users.add( new User( 17, "a" ) );
 users.add( new User( 17, "b" ) );
 users.add( new User( 17, "c" ) );
 users.add( new User( 17, "d" ) );
 UserComparator comparator = new UserComparator();
 
 users.sort( comparator );
 //  You can also use: Collections.sort( users, comparator );
 
 for ( User u : users ) {
 System.out.println( u );
 }
 }
 
 @Override
 public int compare( User u1, User u2 ) {
 if ( u1.equals( u2 ) ) {
 return 0;
 }
 else if ( u1.getAge() < u2.getAge() ) {
 return 1;
 }
 else if ( u1.getAge() == u2.getAge() ) {
 int f = u1.getName().compareTo( u2.getName() );
 if ( f < 0 ) {
  return -1;
 }
 return 0;
 }
 else {
 return -1;
 }
 }
 }
 
 class User {
 private int age;
 private String name;
 
 public User() {
 
 }
 
 public User( int age, String name ) {
 this.age = age;
 this.name = name;
 }
 
 public int getAge() {
 return age;
 }
 
 public void setAge( int age ) {
 this.age = age;
 }
 
 public String getName() {
 return name;
 }
 
 public void setName( String name ) {
 this.name = name;
 }
 
 @Override
 public String toString() {
 return "User [age=" + age + ", name=" + name + "]";
 }
 
 }

User [age=17, name=a]
User [age=17, name=b]
User [age=17, name=c]
User [age=17, name=d]
User [age=15, name=s]
User [age=11, name=d]
User [age=10, name=a]
User [age=6, name=x]