Question

quick question.

Would this always be true?

int i = ...;
double d = i;
if (i == (int) d) ...

Or I need to do rounding to be sure?

if (i == Math.round(d)) ...

Answer 1

Yes, all possible int values can round-trip to a double safely.

You can verify it with this code:

    for (int i = Integer.MIN_VALUE; ; i++) {
        double d = i;
        if (i != (int) d) {
            throw new IllegalStateException("i can't be converted to double and back: " + i);
        }
        if (i == Integer.MAX_VALUE) {
            break;
        }
    }

Note that I’m not using a normal for loop, because it would either skip Integer.MAX_VALUE or loop indefinitely.

Note that the same is not true for int/float or for long/double!