What is the best way to separate double into two parts “integer & fraction” in java

Question

I have tried to separate 5.6 (for example) by the following method:

private static double[] method(double d)
{
    int integerPart = 0;
    double fractionPart =         


        

Answer 1

To see what is going on, take a look at the binary representations of the numbers:

double d = 5.6;
System.err.printf("%016x%n", Double.doubleToLongBits(d));
double[] parts = method(d);
System.err.printf("%016x %016x%n",
                  Double.doubleToLongBits(parts[0]),
                  Double.doubleToLongBits(parts[1]));

output:

4016666666666666
4014000000000000 3fe3333333333330

5.6 is 1.4 * 2², but 0.6 is 1.2 * 2^-1. Because it has a lower exponent, normalization causes the mantissa to be shifted three bits to the left. The fact that the recurring terms (..66666..) were originally an approximation of the fraction 7/5 has been forgotten, and the missing bits are replaced with zeros.

Given the original double value as input to your method, there is no way to avoid this. To preserve the exact value you would need to use a format that represents the desired value exactly, e.g. Fraction from Apache commons-math. (For this specific example with d=5.6 a BigDecimal would also be able to represent it exactly, but there are other numbers it cannot represent exactly, e.g. 4/3)