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Narrower primitive types can be cast to wider types without any effect on the magnitude of numeric values. However, when the expressions are not strictfp, conversions from float to double may lose information about the overall magnitude of the converted value (see JLS Section 5.1.2, Widening Primitive Conversion).

Conversion from int or long to float, or long to double can lead to loss of precision (loss of least significant bits). No runtime exception occurs despite the loss.

Conversion rules:

Noncompliant Code Example

In this noncompliant example, an int is converted to float. Since a floating point number cannot be precise to 9 digits, the result of subtracting the original from this value is non-zero.

class WideSample {
  public static void main(String[] args) {
    int big = 1234567890;
    float approx = big;
    System.out.println(big - (int)approx);  //ideally this should be zero but it prints -46
  }
}

Compliant Solution

The significand part of a floating point number can hold at most 23 bit values. Anything above this threshold is discarded due to precision loss, as is demonstrated in this compliant solution.

class WideSample {
  public static void main(String[] args) {
    int big = 1234567890;
                
    float approx = big;
    if(Integer.highestOneBit(big) <= Math.pow(2, 23)) { //the significand can store at most 23 bits
      System.out.println(big - (int)approx);  //always prints zero now
    }
    else {
      //handle error
      throw new ArithmeticException("Insufficient precision");
    }
  }
}

Risk Assessment

Casting numeric types to wider floating-point types may lose information.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[JLS 05|AA. Java References#JLS 05]\] [Section 5.1.2, Widening Primitive Conversion|http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.1.2]

INT32-J. Perform conversion from BigInteger to String and back properly      04. Integers (INT)      INT34-J. Perform explicit range checking to ensure integer operations do not overflow