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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).

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This noncompliant code example attempts a direct comparison with NaN. 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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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); // Returns NaN when input is infinity
    if (Double.isNaN(result)) { 
      System.out.println("result is NaN");
    }
  }
}

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[FindBugs 2008]	FE: Doomed test for equality to NaN
[JLS 2005]	§4.2.3, "Floating-Point Types, Formats, and Values"
[Seacord 2015]	NUM07-J. Do not attempt comparisons with NaN LiveLesson

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