Finding the least double greater than in Java

Question

Is there a built-in method to find the next greatest double number in Java?

When writing a test, I sometimes want to verify that an operation succeeds for x but fails for values greater than x. One way to do this is to show that the operations fails for x + eps. But eps must be chosen taking into account the value of x to avoid problems like 180 + 1e-15 == 180.

To automatically select eps, I'd like to find the smallest representable number greater than x. I was surprised that I didn't find a standard library method, so I wrote the following:

public static double nextDouble(double d) {
    if (d < 0) {
        return -prevDouble(Math.abs(d));
    } else {
        return Double.longBitsToDouble(Double.doubleToLongBits(d) + 1);
    }
}

public static double prevDouble(double d) {
    if (d < 0) {
        return -nextDouble(-d);
    } else {
        return Double.longBitsToDouble(Double.doubleToLongBits(d) - 1);
    }
}

Should I use these custom functions, or is there a built-in approach available?

Answer 1

There is the Math.nextUp method.

Returns the floating-point value adjacent to d in the direction of positive infinity.

There are overloads to take double and float. There are also corresponding Math.nextDown methods.