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According to the _Java Language Specification_ \[[JLS 2005|AA. Bibliography#JLS 05]\], Section 4§4.2.3, "Floating-Point Types, Formats, and Values" |
NaN(not-a-number) is unordered, so the numerical comparison operators<,<=,>, and>=returnfalseif either or both operands areNaN. The equality operator==returnsfalseif either operand isNaN, and the inequality operator!=returnstrueif either operand isNaN.
Because this unordered property is often unexpected, 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 guideline "NUM11-J. Check floating point inputs for exceptional values" for additional information.
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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, this comparison always return false, and the "result is NaN" message is never printed.
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Automated detection of floating-point comparison operators 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
Bibliography
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NUM09-J. Use the strictfp modifier for floating point calculation consistency across platforms 03. Numeric Types and Operations (NUM) NUM11-J. Check floating point inputs for exceptional values