Java double division positiveness

Question

Why does this java code yield Positive Infinity?

    double d = 10.0 / -0; 

    System.out.println(d);
    if (d == Double.POSITIVE_INFINITY) 
        System.out.println("Positive Infinity");
    else 
        System.out.println("Negative Infinity");

Answer 1

While double distinguishes between positive and negative zero, int does not. So when you convert an int with the value 0 to a double, you always get “positive zero”.

-0 is an int, and it has the value 0.

Divide by “negative zero” to obtain negative infinity. To do so, you need to specify the divisor as a double (not an int):

    double d = 10.0 / -0.0;

    System.out.println(d);
    if (d == Double.POSITIVE_INFINITY) {
        System.out.println("Positive Infinity");
    } else {
        System.out.println("Different from Positive Infinity");
    }
    if (d == Double.NEGATIVE_INFINITY) {
        System.out.println("Negative Infinity");
    } else {
        System.out.println("Different from Negative Infinity");
    }

Output:

-Infinity
Different from Positive Infinity
Negative Infinity

Answer 2

Negative zero is equal to zero. Therefore,

double d = 10.0 / -0;

is equivalent to

double d = 10.0 / 0;

which is positive infinity.

On the other hand, if you instead have:

double d = 10.0 / -0.0;

you will get negative infinity (since now the second value is a double, which differentiates the positive/negative zero).