Comparable & Comparator

TreeSet sorts elements using a Red-Black tree; elements must implement Comparable or a Comparator must be provided, and compareTo returning 0 means the element is a duplicate.

Comparable defines one natural ordering inside the class (compareTo); Comparator defines external orderings outside the class (compare) – use Comparable for the default sort, Comparator for alternate sorts.

Build Comparators using Comparator.comparing() for the primary sort and thenComparing() for tie-breaking, with reversed() for descending and nullsFirst/Last for null safety.

BigDecimal.equals() checks value AND scale (1.0 is not equal to 1.00); compareTo() checks value only (1.0 equals 1.00) – in finance, always use compareTo for equality.