compareTo

compareTo is a method used in programming to determine the relative ordering of two objects. In Java, it is defined by the Comparable interface as int compareTo(T o). The contract states that the method returns a negative integer if this object is less than the specified object, zero if they are equal, and a positive integer if this object is greater.

Implementations should define a natural order for a class. The result of compareTo is used by sorting and ordering operations. For example, Arrays.sort and Collections.sort rely on compareTo to arrange elements, and data structures such as TreeSet and TreeMap use it to maintain order.

A key principle is that compareTo is related to equality but not identical. If compareTo returns zero, many languages expect that equals would also consider the objects equal; however, this is not guaranteed unless equals is implemented consistently with compareTo. Conversely, two objects could be considered equal by equals but not equal in ordering if the class does not define a proper natural order.

Common implementations include lexicographic comparison for strings (based on Unicode values) and numeric comparison for wrapper types like Integer. Many languages offer a separate Comparator to define alternate orderings independent of an object's natural order.

Pitfalls include violating the transitivity or antisymmetry requirements, which can break sorted collections. Null handling varies by language and may throw exceptions if the argument is null.

See also: Comparable, Comparator, sort, equals.