Thanks, glad you found the flaw!

Uuuuggghhh, FINALLY figured it out. It was just because I was returning "Not Applicable" instead of "Not applicable" (needed a lower-case 'a'). That was a tad frustrating :/ My solution worked fine after that. I didn't really consider that I was returning the wrong string because I figured the test case would have said 'Expected: "Not applicable" but was: "Not Applicable"'... :(

1: sys will never be null or empty.
2: product must be greater than... which means > so your test on "product" is not really the good one.
3: verify your "areCoprime". I have no time to test your function but the usual way is with the "gcd" (good exercise: gcd of two numbers and then gcd of a list).
4: if the conditions about product and coprime are not fullfilled there will be no runtime error. You will simply fail the corresponding tests. At last, no test returns something empty.
Keep me informed!

For some reason I am failing test cases Five and Seven when trying to submit in Java. All it says is "expected:but was:"...so I have no idea what it actually expected and what it received, which makes it pretty hard to debug :( It is my understanding that there should be no case where the expected output would be nothing at all (empty string or null).

That being the case, does this mean my code failed the test due to some runtime error?

I have to agree with SithFire here. It specifically says when you hover over the 'Best Practices' button: "Best Practice solutions tend to be a good balance of performance, readability and maintainability."

It does sounds like you are only focusing on performance in your comment.