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Although this noncompliant code example currently works as written, its maintenance is likely to be problematic. If the enum constants were reordered, the getNumberOfCarbon() method would return incorrect values. Furthermore adding an additional BENZENE constant could not be added to the model because it would break the invariant assumed by the getNumberOfCarbon() method; Benzene has 6 carbons, but the ordinal value 6 is already taken by 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 ordinalgetNumberOfCarbon() method is  method no longer required in knowing 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 to be 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;
  }
}

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