which : Java Glossary

A utility that will tell you where on the path some given program is. This helps resolve problems with mulitiple versions of the same program. It also resolves *.com *.exe *.bat confusion. A which command comes with tcc/TakeCommand. You can download which for various platforms. It would be nice if someone wrote a which utility for the classpath as well.

You often want to verify which version of the binary is being used.