FLP10-J. Beware of precision loss when casting primitive integers to floating-point

Narrower primitive types may be cast to wider types without affecting the magnitude of numeric values. See JLS, Section 5.1.2, "Widening Primitive Conversion" for more information. Conversion from int or long to float, or long to double may lead to loss of precision (loss of least significant bits). No runtime exception occurs despite this loss.

Note that conversions from float to double can also lose information about the overall magnitude of the converted value. (See guideline FLP04-J. Use the strictfp modifier for floating point calculation consistency for additional information.)

Noncompliant Code Example

In this noncompliant code example, a value of type int is converted to the type float. Because type float has only 23 mantissa bits, the result of subtracting the original from this value is -46, not zero.

class WideSample {
  public static void main(String[] args) {
    int big = 1234567890;
    float approx = big;

    // This is expected to be zero but it prints -46
    System.out.println(big - (int)approx);  
  }
}

Compliant Solution

Numbers of type float have 23 mantissa bits, a sign bit, and an 8 bit exponent. The exponent allows type float to represent a larger range than that of type int. Nevertheless, integers whose representation requires more than 23 bits can only be represented approximately by a float.

class WideSample {
  public static void main(String[] args) {
    int big = 1234567890;
                  
    // The significand can store at most 23 bits
    if ((big > 0x007fffff) || (big < -0x800000)) { 
      throw new ArithmeticException("Insufficient precision");	
    }

    float approx = big;
    System.out.println(big - (int)approx);  // Prints zero when no precision is lost
  }
}

Risk Assessment

Casting integer values to floating-point types whose mantissa has fewer bits than the original integer value will lose precision.

Guideline	Severity	Likelihood	Remediation Cost	Priority	Level
FLP10-J	low	unlikely	medium	P2	L3

Automated Detection

Automatic detection of casts that can lose precision is straightforward. Sound determination of whether those casts correctly reflect the intent of the programmer is infeasible in the general case. Heuristic warnings could be useful.