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

Wiki MarkupAccording to the Java API \[[API 2006|AA. Bibliography#API 06]\], {{ordinal()}} is defined as {{public final int}} {{{*}ordinal{*}{}}}{{()}}API, Class Enum<E extends Enum<E>> [API 2011], 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.

It defines The Java Language Specification, §8.9, "Enums" [JLS 2013], does not specify the use of ordinal() as a helper function to sophisticated enum-based data-structures EnumSet and EnumMap. Poor understanding of ordinal() can cause programs to behave erroneously 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 noncomplaint noncompliant code example declares enum Hydrocarbon and uses its ordinal() method to provide the result of the getNumberOfCarbons() method.:

enum HydroCarbonHydrocarbon {
  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 worksAlthough this noncompliant code example behaves as expected, its maintenance is susceptible likely to vulnerabilitiesbe problematic. If the enum constants are were reordered, the getNumberOfCarbongetNumberOfCarbons() method does not would return correct incorrect values. Also, if{{BENZENE}}, with 6 carbons, is added to the enum, it cannot be added without violating the current enum designFurthermore, 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 HEXANE.

Compliant Solution

In this compliant solution, we explicitly associate enum constants are explicitly associated with the corresponding integer values .for the number of carbon atoms they contain:

enum HydroCarbonHydrocarbon {
  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;
  }
}

Risk Assessment

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 is permitted with the following enumerated type:

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

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

Related Guidelines

C Secure Coding Standard: INT09-C. Ensure enumeration constants map to unique values

C++ Secure Coding Standard: INT09-CPP. Ensure enumeration constants map to unique values

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

Bibliography

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Image Added Image Added Image Added Wiki Markup\[[JLS 2005|AA. Bibliography#JLS 05]\] Section 8.9, "Enums" \[[API 2006|AA. Bibliography#API 06]\] [Enum|http://download.oracle.com/javase/6/docs/api/java/lang/Enum.html]