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NaN values are particularly problematic because they are unordered. That is, the expression NaN == NaN always returns false. See (see rule NUM07-J. Do not attempt comparisons with NaN for more information).

Noncompliant Code Example

This noncompliant code example accepts user data without validating it.:

double currentBalance; // User's cash balance

void doDeposit(String userInput) {
  double val = 0;
  try {
    val = Double.valueOf(userInput);
  } catch (NumberFormatException e) {
    // Handle input format error
  }

  if (val >= Double.MAX_VALUE - currentBalance) {
    // Handle range error
  }

  currentBalance += val;
}

This code will produce produces unexpected results when an exceptional value is entered for val and subsequently used in calculations or as control values. The user could, for example, input the strings infinity or NaN on the command line, which would be parsed by Double.valueOf(String s) into the floating-point representations of either infinity or NaN. All subsequent calculations using these values would be invalid, possibly causing runtime exceptions or enabling denial-of-service (DoS) attacks.

In this noncompliant example, entering NaN for val would cause currentBalance to be set to NaN, corrupting its value. If this value were used in other expressions, every resulting value would also become NaN, possibly corrupting important data.

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Incorrect or missing validation of floating-point input can result in miscalculations and unexpected results, possibly leading to inconsistent program behavior and denial of service.

Automated Detection

Automated detection is infeasible in the general case. It could be possible to develop a taint-like analysis that detects many interesting cases.

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