NUM11-J. Do not compare or inspect the string representation of floating-point values

String representations of floating-point numbers must not be compared or inspected.

Noncompliant Code Example (String Comparison)

This noncompliant code example compares the string representations of two floating-point values.

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

The comparison unexpectedly fails because s contains the string "0.0010".

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

Comparing or inspecting the string representation of floating-point values may have unexpected results.

Related Vulnerabilities

Hibernate Validator bug report HV-192 describes a violation of this rule.

Android Implementation Details

Comparing or inspecting the string representation of floating-point values may have unexpected results on Android.

Bibliography

      Rule 03: Numeric Types and Operations (NUM)