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According to the Java API, Class Enum<E extends Enum<E>> [API 2011], Class Enum<E extends Enum<E>>, public final int ordinal()

Returns 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 EnumSet and EnumMap.

The Java Language Specification [JLS 2011], §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 getNumberOfCarbons() method.:

enum Hydrocarbon {
  METHANE, ETHANE, PROPANE, BUTANE, PENTANE,
  HEXANE, HEPTANE, OCTANE, NONANE, DECANE;

  public int getNumberOfCarbons() {
    return ordinal() + 1;
  }
}

Although this noncompliant code example works behaves as writtenexpected, its maintenance is likely to be problematic. If the enum constants were reordered, the getNumberOfCarbongetNumberOfCarbons() method would return incorrect values. Furthermore, adding an additional BENZENE constant to the model would break the invariant assumed by the getNumberOfCarbongetNumberOfCarbons() method ; because benzene has 6 six carbons, but the ordinal value 6 is already taken by hexane HEXANE.

Compliant Solution

In this compliant solution, enum constants are explicitly associated with the corresponding integer values for the number of carbon atoms they contain. Consequently, the getNumberOfCarbon() 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 HEXANE and BENZENE. Furthermore, this solution lacks any dependence on the order of the enumeration; the getNumberOfCarbon() method would continue to work even if the enumeration were reordered.:

enum Hydrocarbon {
  METHANE(1), ETHANE(2), PROPANE(3), BUTANE(4), PENTANE(5),
  HEXANE(6), BENZENE(6), HEPTANE(7), OCTANE(8), NONANE(9), 
  DECANE(10);

  private final int numberOfCarbons;

  Hydrocarbon(int carbons) { this.numberOfCarbons = carbons; }

  public int getNumberOfCarbons() {
    return numberOfCarbons;
  }
}

The 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 HEXANE and 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.

Applicability

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 presents no problem is permitted with the following enumerated type:

public enum Day { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, 
                  THURSDAY, FRIDAY, SATURDAY }

However, in In general, use of ordinals to derive integer values reduces the program's maintainability and can lead to errors in the program.

Related Guidelines

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Bibliography

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