Java: Exceptions as control flow?

Generally exceptions are expensive operations and as the name would suggest, exceptional conditions. So using them in the context of controlling the flow of your application is indeed considered bad practice.

Specifically in the example you provided, you would need to do some basic validation of the inputs you are providing to the StringMatch constructor. If it were a method that returns an error code in case some basic parameter validation fails you could avoid checking beforehand, but this is not the case.

I've done some testing on this. On modern JVMs, it actually doesn't impact runtime performance much (if at all). If you run with debugging turned on, then it does slow things down considerably.

See the following for details

(I should also mention that I still think this is a bad practice, even if it doesn't impact performance. More than anything, it reflects a possibly poor algorithm design that is going to be difficult to test)