Possible changes 

Possibilities for discussion.

1. Quads are not extended Triples (useful if the default graph is a union of named graphs but this isn't always the case) but share a common supertype.
1.  Add a CONTRIBUTING file
1.  3 parts of a Literal
1.  getObject is a Resource, not a Literal
Or alternatively support Quads using:
    Map<Graph, Set<Triple>>

where Graph contains isDefault() and getIRI() methods to distinguish between default and named graphs without resorting to relying on (iri == null), also therefore supporting Map implementations that don't allow null keys.