Module system

From APIDesign

Specification and also an implementation for packaging, deploying and executing applications composed from pieces/modules. Java users can choose from OSGi, NetBeans Runtime Container or possibly mix them together.

In case one cares more about the build than execution, then have a look at Maven.

A bit of math

Module system design needs to face some tricky (e.g. NP-Complete) issues. Especially complex is management on module dependencies as described at LibraryReExportIsNPComplete. This can have various solutions (like LibraryWithoutImplicitExportIsPolynomial), but it also shows why good API design skills are needed and why BackwardCompatibility is important.