What happens if I place only a semicolon on a line after a statement??

Answer 1

White space doesn't matter in Java or C++ or PhP.
As long as it doesn't separate a keyword/name/value, it's fine.

Answer 2

makes no diffrence

Answer 3

I'm not sure what you mean, please can you post an example?

Answer 4

Nothing

eg:
if (test)
;
is compilled and nothing happens .