Comparable<T> class Null safety

Interface used by types that have an intrinsic ordering.

The compareTo operation defines a total ordering of objects, which can be used for ordering and sorting.

The Comparable interface should be used for the natural ordering of a type. If a type can be ordered in more than one way, and none of them is the obvious natural ordering, then it might be better not to use the Comparable interface, and to provide separate Comparators instead.

It is recommended that the order of a Comparable agrees with its operator operator == equality (a.compareTo(b) == 0 iff a == b), but this is not a requirement. For example, double and DateTime have compareTo methods that do not agree with operator operator ==. For doubles the compareTo method is more precise than the equality, and for DateTime it is less precise.

Examples:

 (0.0).compareTo(-0.0);  // => 1
 0.0 == -0.0;            // => true
 var dt = new DateTime.now();
 var dt2 = dt.toUtc();
 dt == dt2;              // => false
 dt.compareTo(dt2);      // => 0

The Comparable interface does not imply the existence of the comparison operators <, <=, > and >=. These should only be defined if the ordering is a less-than/greater-than ordering, that is, an ordering where you would naturally use the words "less than" about the order of two elements.

If the equality operator and compareTo disagree, the comparison operators should follow the equality operator, and will likely also disagree with compareTo. Otherwise they should match the compareTo method, so that a < b iff a.compareTo(b) < 0.

The double class defines comparison operators that are compatible with equality. The operators differ from double.compareTo on -0.0 and NaN.

The DateTime class has no comparison operators, instead it has the more precisely named DateTime.isBefore and DateTime.isAfter.