B4J Question have a way to create a java library using B4J ?

You can use the "Compile To Library" option in the Project menu in the B4J IDE. This will compile your project (except for the Main module and other modules labeled #ExcludeFromLibrary) into a library with an .xml and a .jar file. If my recollection serves me, you will find those files in your Additional Libraries folder.

The .jar file can be used as a library in a Java project but you will most likely have to reference jCore.jar in your Java project and possibly other .jar files, as well.

It will not work in most cases as the compiled code is expected to run in a B4J process.