Question

Specifically in Java, how can I determine if a double is an integer? To clarify, I want to know how I can determine that the double does not in fact contain any fractions or decimals.

I am concerned essentially with the nature of floating-point numbers. The methods I thought of (and the ones I found via Google) follow basically this format:

double d = 1.0;
if((int)d == d) {
    //do stuff
}
else {
    // ...
}

I’m certainly no expert on floating-point numbers and how they behave, but I am under the impression that because the double stores only an approximation of the number, the if() conditional will only enter some of the time (perhaps even a majority of the time). But I am looking for a method which is guaranteed to work 100% of the time, regardless of how the double value is stored in the system.

Is this possible? If so, how and why?

Answer 1

double can store an exact representation of certain values, such as small integers and (negative or positive) powers of two.

If it does indeed store an exact integer, then ((int)d == d) works fine. And indeed, for any 32-bit integer i, (int)((double)i) == i since a double can exactly represent it.

Note that for very large numbers (greater than about 2**52 in magnitude), a double will always appear to be an integer, as it will no longer be able to store any fractional part. This has implications if you are trying to cast to a Java long, for instance.