NUM11-J. Check floating point inputs for exceptional values

Floating-point numbers can take on three exceptional values, infinity, -infinity and NaN (not-a-number). These values are produced as a result of exceptional or otherwise unresolvable floating point operations. These exceptional values can also be obtained directly from user input through methods such as Double.valueOf(String s). Failure to detect and handle such exceptional values can result in inconsistent behavior.

NaN values are particularly problematic because they are unordered. That is, the expression NaN == NaN always returns false (see guideline NUM10-J. Do not attempt comparisons with NaN). In general, any comparisons with NaN return false, and all arithmetic functions with one or more NaN inputs produce NaN as their output. Consequently, a single occurrence of a NaN value can cause regressions within other code segments. This correctâ€”and arguably desirableâ€”behavior can cause unexpected results.

The method Double.valueOf(String s) can return NaN or an infinite double, as specified by its contract. Programs should install checks to ensure that all floating point inputs (especially those obtained from the user) are free of unexpected exceptional values. The methods Double.isNaN(double d) and Double.isInfinite(double d) can be used for this purpose.

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;
  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 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.

Compliant Solution

This compliant solution validates the floating point input before using it. The value is tested to ensure that it is neither infinity, -infinity, nor NaN.

double currentBalance; // User's cash balance

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

  if (Double.isInfinite(val)){
    // Handle infinity error
  }

  if (Double.isNaN(val)) {
    // Handle NaN error
  }

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

Exceptions

FLP11-EX1: Occasionally, NaN or infinity may be acceptable as expected inputs to a program. In such cases, explicit checks might not be necessary. However, such programs must be prepared to handle these exceptional values gracefully and should prevent propagation of the exceptional values to other code that fails to handle exceptional values. The choice to permit input of exceptional values during ordinary operation should be explicitly documented.

Risk Assessment

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 (DoS).