Converting a value of type float to the type double is a widening primitive conversion. Refer to the guideline INT33-J. Do not cast numeric types to wider floating-point types without range checking for more details about such conversions. If the value of the float variable must be represented exactly using the double type, an explicit assignment is inappropriate. Note that this is a problem that results from the limited capacity of computers in representing floating point values precisely.
Noncompliant Code Example
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In this noncompliant code example, the programmer attempts to convert the fractional value {{1/3}} (represented by a variable of type {{float}}) to the {{double}} type. The correct representation of this fraction in floating point arithmetic is 0.33333334 (the last digit 4, is rounded up). When converted to {{double}}, instead of observing a value such as 0.3333333400000000 or 0.333333333333333, the programmer obtains an unexpected value because floating point arithmetic in Java _rounds to the nearest_, a mode defined by the IEEE 754 floating point standard. If the exact value lies midway between two valid floating point values, the lower one (with the least significant bit as 0) is chosen. If it is more than midway, a small positive error is introduced, else a small negative error. \[[Mak 02|AA. Java References#Mak 02]\] |
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double d; float f = 1/3f; // Contains the value 0.33333334 d = f; // Now contains the value 0.3333333432674408 |
Compliant Solution
If the exact value must be preserved, the string representation of the floating point number must be obtained. This can be converted to the double type without losing the initial precision.
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double d; float f = 1/3f; // Contains the value 0.33333334 d = Double.valueOf(String.valueOf(f)); // Now contains the value 0.33333334 |
Risk Assessment
Converting from float to double type when exact values are required can affect the precision of the converted value.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
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FLP05- J | low | likely | medium | P6 | L2 |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[API 06|AA. Java References#API 06]\] \[[JLS 05|AA. Java References#JLS 05]\] \[[Mak 02|AA. Java References#Mak 02]\] Section 3.7 "Another Look at Roundoff Errors" |
FLP04-J. Check floating point inputs for exceptional values 07. Floating Point (FLP) FLP30-J. Do not use floating point variables as loop counters