Comparable & Comparator

Ordering: Comparable and Comparator

Comparable<T> is the natural order — the object itself decides:

record Weight(int grams) implements Comparable<Weight> {
    public int compareTo(Weight o) { return Integer.compare(grams, o.grams); }
}

Comparator<T> is an external, swappable order:

Comparator<Employee> bySalary = Comparator.comparingInt(Employee::salary);
Comparator<Employee> bySalaryDesc = bySalary.reversed();
Comparator<Employee> byDeptThenSalary =
    Comparator.comparing(Employee::dept).thenComparingInt(Employee::salary);

Rules of the road:

• compare(a,b) < 0, 0, or > 0 — sign is the only thing that matters
• comparator must be consistent with equals if used in sorted sets/maps, or you get duplicate-looking entries
• Comparator.comparing(keyExtractor) beats hand-written lambdas for readability and null-handling (nullsFirst, nullsLast)

Sorting with streams (sorted(cmp)) and Collections.sort share these interfaces.