基本Java：判断double是否为整数的方法

Question

I'm a beginning programmer and I need a method that returns whether or not a double is an integer. The problem occurs when the number is too big for an int to hold.

Here's what I have:

private static boolean isInteger(double n){
    int ni = (int) n;
    double nd = (double) ni;
    if (nd==n)
        return true;
    return false;
}

Say I put in like 143215890634.0. It will return false because the int can't store that many digits.

How can I allow the int(or another class) to store more digits or is there a better way to determine if a double is an int without comparing like this?

Thanks in advance!

Answer 1

Well long holds more digits than int . By the time you get outside the range of long , all double values are integers (and separated by more than 1). So you could use something like:

private static boolean isInteger(double n) {
    if (n > (double) Long.MAX_VALUE ||  n < (double) Long.MIN_VALUE) {
        return true;
    }
    long asLong = (long) n;
    return n == (double) asLong;
}

There are alternative approaches which would determine the bitwise representation, and check the exponent - but they'd be more complicated, at least to understand.

Note that checking for exact values in binary floating point is usually a bad idea. You might instead want to check how close the double to the nearest integer, and have some sort of tolerance.

Answer 2

try

boolean isInteger(double d) {
    return d % 1 == 0;
}

Answer 3

Compare it to Integer.MAX_VALUE . If it is smaller - it will fit into integer assuming it has no fraction.

Answer 4

To store lager numbers you would have to use long instead. As for the comparison, you could maybe do num == Math.round(num) instead.

Answer 5

I suppose, this would work even better than Math.round(), since it avoids type promotion:

    double d1 = 143215890634.0, d2 = 143215890634.001;
    System.out.println(d1 == Math.floor(d1));
    System.out.println(d2 == Math.floor(d2));

This works, if with "integer" you don't mean actual type "int", but a "number without position after decimal point".