code: public synchronized Set<Artifact> getArtifacts( String groupId, String artifactId, String version )
      {
          Map<String, Map<String, Map<Artifact, Content>>> artifactMap = contents.get( groupId );
          Map<String, Map<Artifact, Content>> versionMap = ( artifactMap == null ? null : artifactMap.get( artifactId ) );
          Map<Artifact, Content> filesMap = ( versionMap == null ? null : versionMap.get( version ) );
  
          return new HashSet<Artifact>( filesMap == null ? Collections.<Artifact>emptySet() : filesMap.keySet() );
      }
docs: {@inheritDoc}