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Wiki MarkupAccording to the The Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], Section 4 (JLS), §4.2.3, "Floating-Point Types, Formats, and Values"[JLS 2015]:

NaN (not-a-number) is unordered, so the numerical comparison operators <, <=, >, and >= return false if either or both operands are NaN. The equality operator == returns false if either operand is NaN, and the inequality operator != returns true if either operand is NaN.

Because this " unordered property " is often unexpected, problems direct comparisons with NaN must not be performed. Problems can arise when programmers write code that compares floating-point values without considering the semantics of NaN. For example, input validation checks that fail to consider the possibility of a NaN value as input may can produce unexpected results (see NUM08-J. Check floating-point inputs for exceptional values for additional information).

Noncompliant Code Example

This noncompliant code example attempts a direct comparison with NaN. As per In accordance with the semantics of NaN, all comparisons with NaN yield false (with the exception of the != operator, which returns true). Consequently, the this comparison must always return false, and the "result is NaN" message is never printed.

public class NaNComparison {
  public static void main(String[] args) {
    double x = 0.0;
    double result = Math.cos(1/x); // returnsReturns NaN if input is infinity
    if (result == Double.NaN) { // comparisonComparison is always false!
      System.out.println("result is NaN");
    }
  }
}

Compliant Solution

This compliant solution uses the method Double.isNaN() to check whether the expression corresponds to a NaN value.:

public class NaNComparison {
  public static void main(String[] args) {
    double x = 0.0;	  
    double result = Math.cos(1/x); // returnsReturns NaN when input is infinity
    if (Double.isNaN(result)) { 
      System.out.println("result is NaN");
    }
  }
}

Risk Assessment

Comparisons with NaN values can lead to unexpected results.

Automated Detection

Automated detection of floating point comparison operators with NaN is straightforward. Sound determination of whether the possibility of an unordered result has been correctly handled is not feasible in the general case. Heuristic checks could be useful.

Findbugs checks for the specific case of comparison with a constant NaN.

Related Vulnerabilities

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

Bibliography

\[[FindBugs 2008|AA. Bibliography#FindBugs 08]\] FE: Doomed test for equality to NaN
\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 4.2.3, Floating-Point Types, Formats, and Values|http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.2.3]

Bibliography

[FindBugs 2008]	FE: Doomed test for equality to NaN
[JLS 2015]	§4.2.3, "Floating-Point Types, Formats, and Values"
[Seacord 2015]	Image Added NUM07-J. Do not attempt comparisons with NaN LiveLesson

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Image Added Image Added Image AddedNUM04-J. Use the strictfp modifier for floating point calculation consistency      03. Floating Point (FLP)      NUM06-J. Check floating point inputs for exceptional values