Comparable vs Comparator

• Comparable<T> defines a class's natural order inside the class itself, through compareTo. Collections.sort(list), TreeSet and TreeMap use it by default. There is only one natural order.
• Comparator<T> is a separate ordering object, so you can have as many orders as you like without touching the class.

record Student(String name, double cgpa) implements Comparable<Student> {
    public int compareTo(Student o) { return name.compareTo(o.name); }    // natural order: by name
}

List<Student> list = new ArrayList<>(List.of(
    new Student("Ravi", 8.4), new Student("Asha", 9.1), new Student("Dev", 8.4)));

list.sort(Comparator.comparingDouble(Student::cgpa).reversed()
                    .thenComparing(Student::name));
// Asha 9.1, Dev 8.4, Ravi 8.4

Avoid the old trick return a.score - b.score; — subtraction can overflow for large or negative values. Use Integer.compare(a, b) or the Comparator.comparing… helpers. Ideally compareTo should be consistent with equals, because sorted sets treat compareTo == 0 as "duplicate".