Comparable & Comparator

Comparable gives a class one natural ordering; Comparator defines any number of external, situational orderings without changing the class itself.

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

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

    @Override
    public int compareTo(Person other) {
        return Integer.compare(this.age, other.age);
    }
}

List<Person> people = new ArrayList<>();
people.add(new Person("Ada", 36));
people.add(new Person("Grace", 85));

Collections.sort(people); // uses compareTo — sorts by age

people.sort(Comparator.comparing(p -> p.name)); // sorts by name instead

Common mistakes

• Implementing compareTo inconsistently with equals (e.g. two objects compare as "equal" via compareTo but .equals() says they're different) — this can produce surprising results in sorted collections like TreeSet.