Don't set a system CLASSPATH unless you have problems accessing your work. Set a CLASSPATH for the application with the -cp or similar options. Look for the details of the "java" and "javac" tools (these are the Windows versions).
To answer the original question, set CLASSPATH *creates* a classpath variable, whether or not one already existed.
Joined: Oct 13, 2005
Presumably, however, if you set the classpath with that instruction at the command line, that classpath variable will only be valid for the lifetime of that command window, and will vanish when the command prompt is closed. That is much safer than setting a system classpath.