FLP02-J. Do not attempt comparisons with NaN

According to [JLS Section 4.2.3, Floating-Point Types, Formats, and Values]:

"NaN 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."

Problems can ensue when the programmer uses such operators on NaN values in comparison operations. There is also a possibility that the input validation condition does not expect a NaN value as input.

Noncompliant Code Example

A frequently encountered mistake is the doomed comparison with NaN, typically in expressions. As per its semantics, no value can be compared to NaN using common operators, including NaN itself. This noncompliant example demonstrates one of such cases.

public class NaNComparison {
  public static void main(String[] args) {
    double x = 0.0;
    double result = Math.cos(1/x); // returns NaN if input is infinity
    if(result == Double.NaN) // compare with infinity
      System.out.println("Both are equal");
  }
}

Compliant Solution

This compliant solution uses Double.isNaN to check if 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 if input is infinity
    if(Double.isNaN(result)) 
      System.out.println("Both are equal");
  }
}

Risk Assessment

Comparisons with NaN values may lead to unexpected results.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] Section 4.2.3, Floating-Point Types, Formats, and Values
[[FindBugs 08]] FE: Doomed test for equality to NaN

FLP01-J. Take care in rearranging floating point expressions      05. Floating Point (FLP)      FLP30-J. Do not use floating point variables as loop counters