Enthuware

I would have thought that since the code that prints "4" would be unreachable, this would cause the code to not compile. I guess the compiler is not able to figure that out because the value of flag is not known until run time?

That is correct. flag is a variable and so its value is not known to the compiler. But even if it were a constant, the code would compile because JLS permits an unreachable if block to support conditional compilation. Please see Section 14.21 (towards the end) for details.

HTH,
Paul.