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Conversion from int or long to float or from long to double can lead to loss of precision (loss of least significant bits). In this casethese cases, the resulting floating-point value is a rounded version of the integer value, using IEEE 754 round-to-nearest mode. Despite this loss of precision, the JLS requires that the conversion and rounding occur silently, that is, without any runtime exception. See the JLS, §5.1.2, "Widening Primitive Conversion" for more information. Conversions from integral types smaller than int to a floating-point type and conversions from int to double can never result in a loss of precision. Consequently, programs must ensure that conversions from an int or long to a floating-point type, or from long to double do not result in a loss of required precision.

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This method could have unexpected results because of the loss of precision. Values In FP-strict mode, values of type float have 23 mantissa bits, a sign bit, and an 8-bit exponent. See rule NUM06-J for more information about FP-strict mode. The exponent allows type float to represent a larger range than that of type int. However, the 23-bit mantissa means that float supports exact representation only of integers whose representation fits within 23 bits; float supports only approximate representation of integers outside that range.

strictfp class WideSample {
  public static int subFloatFromInt(int op1, float op2) {
    return op1 - (int)op2;
  }

  public static void main(String[] args) {
    int result = subFloatFromInt(1234567890, 1234567890);
    // This prints -46, and not 0 as may be expected
    System.out.println(result);  
  }

}

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This compliant solution range checks the argument of the integer argument (op1) to ensure it can be represented as a value of type float without a loss of precision.

strictfp class WideSample {
  public static int subFloatFromInt(int op1, float op2)
                    throws ArithmeticException {

    // The significand can store at most 23 bits
    if ((op1 > 0x007fffff) || (op1 < -0x800000)) { 
      throw new ArithmeticException("Insufficient precision");
    }

    return op1 - (int)op2;
  }

  public static void main(String[] args) {
    int result = subFloatFromInt(1234567890, 1234567890);
    System.out.println(result);  
  }
}

In this example, the subFloatFromInt() method throws ArithmeticException. This general approach, with appropriate range checks, should can be used for conversions from long to either float or double.

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This compliant solution accepts an argument of type double instead of an argument of type float. Values In FP-strict mode, values of type double have 52 mantissa bits, a sign bit, and an 11-bit exponent. Integer values of type int and narrower can be converted to double without a loss of precision.

strictfp class WideSample {
  public static int subDoubleFromInt(int op1, double op2) {
    return op1 - (int)op2;
  }

  public static void main(String[] args) {
    int result = subDoubleFromInt(1234567890, 1234567890);
    // Works as expected
    System.out.println(result);  
  }

}

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