NUM14-J. Do not rely on the default string representation of floating-point values

Working with string representations of floating-point values can produce incorrect conclusions about the precision of the values. For example, consider the conversion of a value from type float to type double, a widening primitive conversion. Refer to the rule "NUM17-J. Beware of precision loss when converting primitive integers to floating-point" for more details about such conversions.

String representations of floating-point numbers shall not be compared or inspected. When the value of a float variable must be represented exactly using the double type, an explicit assignment is more appropriate than first converting the floating-point value to a String and then to a double.

Noncompliant Code Example (String Comparison)

This noncompliant code example attempts a string-based comparison of a floating-point number.

int i = 1;
String s = Double.valueOf(i / 1000.0).toString();
if (s.equals("0.001")) {
  // ...
}

However s actually contains the string "0.0010". Consequently, the comparison unexpectedly fails.

Noncompliant Code Example (Regex)

This noncompliant code example attempts to mitigate the extra trailing zero by using a regular expression on the string before comparing it.

int i = 1;
String s = Double.valueOf(i / 1000.0).toString();
s = s.replaceFirst("[.0]*$", "");
if (s.equals("0.001")) {
  // ...
}

While the comparison does succeed on the code above, it fails on the similar code below, which uses 1/10000.0 instead of 1/1000.0. The string produced is not 0.00010 but rather 1.0E-4.

int i = 1;
String s = Double.valueOf(i / 10000.0).toString();
s = s.replaceFirst("[.0]*$", "");
if (s.equals("0.0001")) {
  // ...
}

Compliant Solution (String Comparison)

This compliant solution uses the BigDecimal class to avoid precision loss. It then performs a numeric comparison, which passes as expected.

int i = 1;
BigDecimal d = new BigDecimal(Double.valueOf(i / 1000.0).toString())
if (d.compareTo(new BigDecimal("0.001")) == 0) {
  // ...
}

Risk Assessment

Relying on the string representation of floating-point types can result in imprecise values.

Automated Detection

TODO

Related Vulnerabilities

HV-192

Bibliography

NUM13-J. Do not construct BigDecimal objects from floating-point literals      03. Numeric Types and Operations (NUM)      NUM15-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data