Comparable vs Comparator — multiple sort orders?

Comparable vs Comparator

Comparable<T> is the natural order inside the type (compareTo). Comparator<T> is an external strategy—multiple sort orders without mutating the class.

• Implement Comparable when one obvious order exists (IDs, timestamps).
• Use Comparator.comparing / thenComparing for fluent multi-field sorts.
• Consistency with equals is recommended but not required—document when unequal objects compare equal.
• Tim sort exploits existing order in partially sorted data.

List<Employee> staff = ...;
staff.sort(Comparator.comparing(Employee::dept)
    .thenComparing(Employee::salary, Comparator.reverseOrder()));