From the entry point, stub a function to throw an exception. You'll reach it. Unit testing isn't about finding every path. Nor is it about writing unit tests for every possible combination of values in a program. A java function that uses an int does not need to test -2147483648 to 2147483647 as inputs. That's not helpful.
That’s an interesting example, that’s exactly where you’re likely to find bugs - at boundary values. You should test those more than you should test the number 5.
This is exactly why unit testing gives a false sense of security. You’ve done a lot, you’ve written all kinds of tests - but at the end of the day, you don’t get a proportionate amount of confidence about the code because there are infinite more cases that you haven’t thought about.
In Java, if my function doesn't modify the int, there's no reason to test the boundaries, other than 0 if the int is used in a calculation. The type system doesn't solely determine what tests to write. Tests are inferred from a combination of type system, timings, statements and variable usage.