How does Comparable's compareTo() method relate to equals(), and what does it mean for compareTo() to be 'consistent with equals'?

compareTo() defines a class's natural ordering (used by sorting and by TreeMap/TreeSet), and being 'consistent with equals' means x.compareTo(y) == 0 should hold if and only if x.equals(y) is true -- this consistency is strongly recommended but not strictly enforced by the compiler, and violating it (as seen with a TreeSet using only salary-based compareTo() while equals() compares all fields) produces genuinely confusing behavior since TreeSet/TreeMap use compareTo() (not equals()) to determine element uniqueness, meaning such a collection's behavior would silently diverge from a HashSet/HashMap using the same class's equals()/hashCode(), a subtle but important consistency trap.

public class Employee implements Comparable<Employee> {
    private final int salary;
    private final String name;

    @Override
    public int compareTo(Employee other) {
        int result = Integer.compare(salary, other.salary);
        if (result != 0) return result;
        return name.compareTo(other.name);  // tiebreaker ensures compareTo()==0 implies genuinely equal
    }
    // equals()/hashCode() should compare the SAME fields (salary AND name) for true consistency
}