java-comparable-and-comparator

Sorting and ordered collections (TreeSet, TreeMap) need to determine the relative order of objects. Without a standard way to compare objects, the JVM would have no idea how to sort a list of custom objects or maintain order in a sorted collection.

Comparable defines a natural ordering for objects of a class — the class implements Comparable<T> and overrides compareTo(). Comparator is a separate interface for defining custom orderings — useful when you need multiple sorting strategies or cannot modify the class.

Comparable.compareTo(other) returns negative (this < other), zero (this other), or positive (this > other). Sorted collections (TreeSet, TreeMap) and utility methods (Collections.sort(), Arrays.sort()) use compareTo() by default. A Comparator can be passed to override the natural order.

• Consistent with equals: Natural ordering should be consistent with equals (or document if not)
• Comparator methods (Java 8+): Comparator.comparing(), thenComparing(), reversed() for fluent construction
• null handling: Comparators can handle nulls via nullsFirst() and nullsLast()
• Sorting stability: Java’s sorting algorithms (TimSort, Dual-Pivot QuickSort) are stable

• Built from: Java Collections Framework — sorted collections require comparison
• Builds into: Java Collections Framework — TreeSet and TreeMap use Comparable/Comparator for sorting
• Contrasts with: Java Iterator — Comparable defines ordering; Iterator defines traversal

• compareTo must be transitive: If a > b and b > c, then a > c must hold
• compareTo must be reflexive: a.compareTo(a) must return 0
• compareTo consistency with equals: Inconsistent classes break Set/Map contracts
• compare returns int: Overflow risk when subtracting values — use Integer.compare() instead