it is impossible to enforce dependencies. With Lookup, a module’s code cannot request an object of type X unless it can load/access the X.class. The module trying to look up an X will not be able to see X.class unless it declares a dependency on the module that defines X and the module which defines X says that it allows access to the Java package X lives in. (A Map<Class<T>,T> would do the same job as Lookup.)

The class of values in a map can change without notice - so if you have (SomeIface) foo = (SomeIface) globalMap.get("foo"), some new version of the module that provides "foo" can change the return type, causing ClassCastException s; with Lookup, you cannot ever get an object that is not of the type you passed in the request - so Lookup’s approach is more robust.

Lookup supports listening to changes in the result.

Lookup supports multiple instances for one key - if you call lookup(X.class) you get one instance. If you call lookupAll(X.class) you get a Collection<? extends X> (so with lookupAll() it is more like a Map<Class<T>,List<T>>)