The Comparable interface defines a single method, compareTo, which enables instances of a class to declare a natural ordering. Unlike methods inherited from Object, compareTo suports comparative logic beyond equality—allowing sorting, binary search, and ordered collection usage without external comparators.

Sorting an array of comparable elements requires no custom logic:

String[] words = {"zebra", "apple", "banana"};
Arrays.sort(words); // Results in ["apple", "banana", "zebra"]

Similarly, constructing a sorted set automatically maintains order and uniqueness:

public class SortedWordDeduplicator {
    public static void main(String[] args) {
        Set<String> uniqueSorted = new TreeSet<>(List.of(args));
        System.out.println(uniqueSorted);
    }
}

Classes that represent values with intuitive orderings—such as numbers, dates, or lexicographic strings—should implement Comparable<T> to integrate seamlessly with Java’s generic collections and algorithms. Nearly all JDK value types (Integer, BigDecimal, LocalDateTime, enums, etc.) conform to this practice.

The interface signature is:

public interface Comparable<T> {
    int compareTo(T other);
}

The compareTo contract mandates three core properties:

• Symmetry of sign: For any non-null x and y, Math.signum(x.compareTo(y)) must equal -Math.signum(y.compareTo(x)). If one throws ClassCastException, the other must too.
• Transitivity: If x.compareTo(y) > 0 and y.compareTo(z) > 0, then x.compareTo(z) > 0 must hold.
• Consistency with equality semantics: If x.compareTo(y) == 0, then for all z, x.compareTo(z) and y.compareTo(z) must have identical sign (both negative, zero, or positive).

While not strictly enforced, it’s strongly advised that compareTo align with equals: (a.compareTo(b) == 0) == a.equals(b). When this invariant is broken—e.g., case-insensitive string wrappers that ignore case in comparison but retain case-sensitive equality—the class should document the divergence explicitly, such as with: "Note: Natural ordering differs from equals behavior."