Java language enumeration types have an
ordinal() method that returns the numerical position of each enumeration constant in its class declaration.
returns the ordinal of the enumeration constant (its position in its enum declaration, where the initial constant is assigned an ordinal of zero). Most programmers will have no use for this method. It is designed for use by sophisticated enum-based data structures, such as
The Java Language Specification, §8.9, "Enums" [JLS 2013], does not specify the use of
ordinal() in programs. However, attaching external significance to the
ordinal() value of an
enum constant is error prone and should be avoided for defensive programming.
Noncompliant Code Example
This noncompliant code example declares
enum Hydrocarbon and uses its
ordinal() method to provide the result of the
Although this noncompliant code example behaves as expected, its maintenance is likely to be problematic. If the
enum constants were reordered, the
getNumberOfCarbons() method would return incorrect values. Furthermore, adding an additional
BENZENE constant to the model would break the invariant assumed by the
getNumberOfCarbons() method because benzene has six carbons, but the ordinal value 6 is already taken by
In this compliant solution,
enum constants are explicitly associated with the corresponding integer values for the number of carbon atoms they contain:
getNumberOfCarbons() method no longer uses the
ordinal() to discover the number of carbon atoms for each value. Different
enum constants may be associated with the same value, as shown for
BENZENE. Furthermore, this solution lacks any dependence on the order of the enumeration; the
getNumberOfCarbons() method would continue to work even if the enumeration were reordered.
It is acceptable to use the ordinals associated with an enumerated type when the order of the enumeration constants is standard and extra constants cannot be added. For example, the use of ordinals is permitted with the following enumerated type:
In general, use of ordinals to derive integer values reduces the program's maintainability and can lead to errors in the program.
|[Bloch 2008]||Item 31, "Use Instance Fields Instead of Ordinals"|
|[JLS 2013]||§8.9, "Enums"|