License Key Generation in Java

Two questions there.

The licence key problem is easy, decide on HOW the licence key will be checked, for example summing the digits adds up to 93, then write a simple function to generate them. Personally, I use a hash code from the registration details, e.g. the user's email.

To make an exe is not trivial. Most apps that appear as exe's just load the jar, a pretty simple step you can search for, try "launch a jar from C#" or whatever language you want to put the launcher in.

InnoSetup[^]

for creating installer.

Otherwise you can simply export as "runnable jar" and add a shortcut to run it.
You can also bundle the JVM with it, so you would be sure the environment is included and your app will run.

Just check out the export-options of your IDE.