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According to the _Java Language Specification_ \[[JLS 2005|AA. Bibliography#JLS 05]\], [§4.2.3, "Floating-Point Types, Formats, and Values"|http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.2.3] \[[JLS 2005|AA. Bibliography#JLS 05]\]: 

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, 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 can produce unexpected results. See rule NUM08-J . Check floating-point inputs for exceptional values for additional information.

Noncompliant Code Example

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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.

Bibliography

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