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Avoid using the primitive floating point types when precise computation is necessary, especially when performing currency calculations. Instead, consider alternative representations that are able to completely represent the necessary values. Whatever representation you choose, you must carefully and methodically estimate the maximum cumulative error of the computations to ensure that the resulting error is within acceptable tolerances. Consider using numerical analysis to properly understand the problem. See \[[Goldberg 1991|AA. Bibliography#Goldberg 91]\] for an introduction to this topic.

Noncompliant Code Example

This noncompliant code example performs some basic currency calculations.

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Because the value 1/10 lacks an exact representation in either Java floating point type — and, indeed, in (or any floating point format that uses a binary mantissa — ) this program prints

A dollar less 7 dimes is $0.29999999999999993

Compliant Solution

This compliant solution uses an integer type (such as long) and works with cents rather than dollars.

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A dollar less 7 dimes is 30 cents

Compliant Solution

This compliant solution uses the BigDecimal type which provides exact representation of decimal values. Note that on most platforms computations performed using BigDecimal are less efficient than those performed using primitive types. The importance of this reduced efficiency is application-specific.

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A dollar less 7 dimes is $0.3

Risk Assessment

Using a floating point representation can result in a loss of precision and accuracy when precise computation is required.

Automated Detection

Automated detection of floating point arithmetic is straight-forward; determining which code suffers from insufficient precision is not feasible in the general case. Heuristic checks, such as flagging floating point literals that cannot be represented precisely, may be useful.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Related Guidelines

C Secure Coding Standard: FLP02-C. Avoid using floating point numbers when precise computation is needed

C++ Secure Coding Standard: FLP02-CPP. Avoid using floating point numbers when precise computation is needed

Bibliography

\[[Bloch 2008|AA. Bibliography#Bloch 08]\] Item 48: Avoid {{float}} and {{double}} if exact answers are required
\[[Bloch 2005|AA. Bibliography#Bloch 05]\] Puzzle 2: Time for a Change
\[[Goldberg 1991|AA. Bibliography#Goldberg 91]\]
\[[IEEE 754|AA. Bibliography#IEEE 754 2006]\]
\[[JLS 2005|AA. Bibliography#JLS 05]\] [Section 4.2.3|http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.2.3], "Floating-Point Types, Formats, and Values"

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