Comparable and Comparator

Definition

Comparable gives a class its natural order through compareTo. A Comparator defines a different order from the outside, so you can sort the same objects in several ways. Comparator.comparing builds one from a field.

Example code

Natural order, then custom orders

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

class Person implements Comparable<Person> {
    final String name;
    final int age;

    Person(String name, int age) { this.name = name; this.age = age; }

    @Override public int compareTo(Person other) { return name.compareTo(other.name); }   // natural order: by name
    @Override public String toString() { return name + "(" + age + ")"; }
}

class Demo {
    public static void main(String[] args) {
        List<Person> people = new ArrayList<>(List.of(new Person("Ravi", 30), new Person("Asha", 30), new Person("Meera", 25)));

        people.sort(null);                                                   // uses compareTo
        System.out.println(people);

        people.sort(Comparator.comparingInt((Person p) -> p.age).thenComparing(p -> p.name));   // age, then name
        System.out.println(people);

        people.sort(Comparator.comparingInt((Person p) -> p.age).reversed());                    // oldest first
        System.out.println(people);
    }
}

// Output:
// [Asha(30), Meera(25), Ravi(30)]
// [Meera(25), Asha(30), Ravi(30)]
// [Asha(30), Ravi(30), Meera(25)]