ComparableとComparatorの違いは何ですか？

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

    @Override
    public int compareTo(Person other) {       // defines THE natural order
        return Integer.compare(this.age, other.age);  // by age
    }
    // return: negative if this < other, 0 if equal, positive if this > other
}

List<Person> people = ...;
Collections.sort(people);   // uses compareTo — sorts by age (the natural order)

// define orderings OUTSIDE the class — as many as you need
Comparator<Person> byName = (a, b) -> a.getName().compareTo(b.getName());
Comparator<Person> byAge = Comparator.comparingInt(Person::getAge);

people.sort(byName);                          // sort by name
people.sort(byAge.reversed());                // by age, descending

Comparator<Person> comp = Comparator
    .comparing(Person::getLastName)            // primary: last name
    .thenComparing(Person::getFirstName)        // tie-break: first name
    .thenComparingInt(Person::getAge)           // then: age
    .reversed();                                 // reverse the whole thing

people.sort(comp);

Comparable  → the class has ONE obvious natural order (numbers by value, strings
              alphabetically, dates chronologically); you own the class
Comparator  → you need MULTIPLE orderings, situational/custom ordering, OR you
              can't modify the class (e.g. sorting a third-party type differently)