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String representations of floating-point numbers must should not be compared or inspected. If they are used, significant care needs to be taken to ensure expected behavior.

Noncompliant Code Example (String Comparison)

This noncompliant code example incorrectly 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", which is not equal to "0.001".

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")) {
  // ...
}

Although the comparison succeeds on the preceding code, it fails on the following similar code, which uses 1/10000.0 instead of 1/1000decimal string literal generated by 1/10000.0. The string produced is not 0.00010 but 0001 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")) {
  // ...
}

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This compliant solution uses the BigDecimal class to avoid precision lossthe conversion into scientific notation. It then performs a numeric comparison, which passes as expected.

int i = 1;
BigDecimal d = new BigDecimal(Double.valueOf(i / 100010000.0).toString());
if (d.compareTo(new BigDecimal("0.0010001")) == 0) {
  // ...
}

Risk Assessment

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

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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

[API 2006]

 

[JLS 2015]

 

[Seacord 2015]	Image Modified NUM11-J. Do not compare or inspect the string representation of floating-point values LiveLesson

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