Comparable Interface
If you want to sort some user defined Objects, then you need to implement interface Comparable or Comparator

    class Student implements Comparable{
        public String name;
        public int compareTo(Student other){
            // 3 2 1
            return  - name.compareTo(other.name);
        }
        
        public Student(String name){
            this.name = name;
        }
        public void print(){
            Print.p("name=" + name);
        }
    }

Comparator Interface

    class Person{
        public String name;
        public Person(String name){
            this. name = name;
        }
        public void print(){
            Print.p("name=" + name);
        }
    }

    class ComPerson implements Comparator{
        public int compare(Person s1, Person s2){
            return s1.name.compareTo(s2.name); 
        }
    }

        {
            List list = new ArrayList<>();
            list.add(new Student("David"));
            list.add(new Student("Michael"));
            Collections.sort(list);

            for(Student s : list){
                s.print();
            }
        }
        {
            List list = new ArrayList<>();
            list.add(new Person("David"));
            list.add(new Person("Michelle"));
            list.add(new Person("Joyce"));
            Collections.sort(list, new ComPerson());
            Print.fl();

            for(Person s : list){
                s.print();
            }
        }