users.sort(new Comparator<User>() {
    public int compare(User u1, User u2) {
        if (u1.getAge() == u2.getAge())
            return 0;
        return u1.getAge() < u2.getAge() ? -1 : 1;
    }
});

Practical example

In this example, we create a custom Comparator that will help us to sort users ArrayList by users age.

import java.util.*;

public class Example {

    public static void main(String[] args) {
        User user1 = new User(1, "Tom", 24);
        User user2 = new User(2, "Kate", 22);
        User user3 = new User(3, "Ann", 25);

        List<User> users = new ArrayList<>();

        users.add(user1);
        users.add(user2);
        users.add(user3);

        System.out.println("Before: " + users);

        users.sort(new Comparator<User>() {
            public int compare(User u1, User u2) {
                if (u1.getAge() == u2.getAge())
                    return 0;
                return u1.getAge() < u2.getAge() ? -1 : 1;
            }
        });

        System.out.println("After:  " + users);
    }
}

Output:

Before: [User{ id=1, name='Tom', age=24 }, User{ id=2, name='Kate', age=22 }, User{ id=3, name='Ann', age=25 }]
After:  [User{ id=2, name='Kate', age=22 }, User{ id=1, name='Tom', age=24 }, User{ id=3, name='Ann', age=25 }]

Note:

We are able to print users in the Example class, because we have overridden toString() method in the User class.

User class:

public class User {
    private final int id;
    private final String name;
    private final int age;

    public User(int id, String name, int age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "User{ " +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                " }";
    }
}