Overflow-safe way to calculate percentage from two longs in Java

Question

I have two non-negative longs. They may be large, close to Long.MAX_VALUE. I want to calculate a percentage from the two numbers.

Usually I'd do this:

    long numerator = Long.MAX_VALUE / 3 * 2;
    long denominator = Long.MAX_VALUE;

    int percentage = (int) (numerator * 100 / denominator);
    System.out.println("percentage = " + percentage);

This is not correct if numerator is within two order of magnitudes to Long.MAX_VALUE.

What's a correct, simple, and fast way to do this?

Answer 1

I'd use:

int percentage = (int)(numerator * 100.0 / denominator + 0.5);

The 100.0 forces floating-point math from that point on, and the + 0.5 rounds to the nearest integer instead of truncating.