DCL11-J. Never derive a value associated with an enum from its ordinal

Java 1.5 supports the use of enumerated types; these enums look just like their C and C++ counterparts. In the Java programming language, however, enums are far more powerful than their counterparts in other languages. In Java, all enums have an ordinal() method, which returns the numerical position of each enum constant in its class declaration.

The Java Language Specification, in Section 8.9, "Enums" does not specify the use of ordinal() in programs. However, improper use of ordinal() method in program logic can cause errors in programs.

According to the Java API [[API 2006]], ordinal() is defined as

public final int ordinal()

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 ordinal() function is also a helper function to sophisticated enum-based data-structures EnumSet and EnumMap.

Poor program design involving ordinal() can cause programs to behave erroneously.

Noncompliant Code Example

This noncomplaint 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;
  }
}
public class TestHC {
  public static void main(String args[]) {
    /* ... */
    HydroCarbon hc = HydroCarbon.HEXANE;
    int index = hc.getNumberOfCarbons();
    int noHyd = NumberOfHydrogen[index]; // Can cause ArrayIndexOutOfBoundsException
  }
}

While the enum code above works, its maintenance is susceptible to vulnerabilities. If the enum constants were to be reordered, the getNumberOfCarbon() method would not return correct values. Also, BENZENE — which also has 6 carbons — cannot be added without violating the current enum design.

Compliant Solution

In this compliant solution, we explicitly associate enum constants with corresponding integer values.

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;
  }
}