List<MavenArtifact> (@Nullable String groupId, @Nullable String artifactId) { Map<String, List<MavenArtifact>> groupMap = myData.get(groupId); if (groupMap == null) return Collections.emptyList(); List<MavenArtifact> res = groupMap.get(artifactId); return res == null ? Collections.emptyList() : res; }