DCL11-J. Do not derive a value associated with an enum from its ordinal

Java language enumeration types have an ordinal() method, which returns the numerical position of each enumeration constant in its class declaration.

Section 8.9, "Enums" of the Java Language Specification [[JLS 2005]] does not specify the use of ordinal() in programs. The ordinal() method relies on the placement of an enum constant in the list (which may be altered in the future due to insertions or deletions) rather than on an inherent property of the constant. Instead, use the rich enum features of Java to associate properties with each enum constant.

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.

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;
  }
}

While this noncompliant code example works, its maintenance is susceptible to vulnerabilities. If the enum constants were reordered, the getNumberOfCarbon() method would return incorrect values. Also the current enum design can not accommodate the addition of BENZENE, which also has 6 carbons.

Compliant Solution

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

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

  private final int numberOfCarbons;

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

  public int getNumberOfCarbons() {
    return numberOfCarbons;
  }
}

In this example, it is trivial to add BENZENE(6) to the list of constants with no fear of errors.

Risk Assessment

Use of ordinals to derive integer values reduces the program's maintainability and can lead to errors in the program.

Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
DCL11-J	low	probable	medium	P4	L3

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

[[JLS 2005]] Section 8.9, "Enums"
[[API 2006]] Enum

DCL10-J. Ensure proper initialization by declaring class and instance variables final Declarations and Initialization (DCL) 04. Expressions (EXP)

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