The use of modules is encouraged but not required, this allows existing code to continue working in Java 9. It also allows for a gradual transition to modular code.
Any non-modular code is put in an unnamed module when it is compiled. This is a special module that is able to use types from all other modules but only from packages which have an
All packages in the unnamed module are exported automatically.
module etc..., are restricted in use within the module declaration but can be continue to be used as identifiers elsewhere.