It would be useful for the jar tool to report the automatic module name that would be used if a regular jar file was put on the module path ( so there is an easy way for developers to determine the derived name ).

One way to achieve this is to repurpose the somewhat fingersome '--print-module-descriptor’ option, to report the automatic module name if the jar file does not contain a module-info.class. Given this, we should then probably give the option a better name, say ‘--describe-module’ ( similar to that of jmod's ‘describe’ subcommand ), e.g.

 $ touch resource
 $ jdk/bin/jar --create --file foo-6.jar resource
 $ jdk/bin/jar --describe-module --file foo-6.jar
 No module descriptor found. Derived automatic module: foo@6

It is also useful for the jar tool to indicate when a jar is not suitable to use as an automatic module, e.g.

 $ jdk/bin/jar --create --file ttt.jar T.class
 $ jdk/bin/jar --describe-module --file ttt.jar
 Unable to derive module descriptor for: ttt.jar
 T.class found in top-level directory (unnamed package not allowed in module)