code: private String groupByProjectModelFunction(final GraphContext context, final Vertex vertex)
      {
          JavaSourceFileModel javaModel = context.getFramed().frameElement(vertex, JavaSourceFileModel.class);
          // String that identifies 3 properties - projectModel + packageName + className that must be the same for vertices
          ProjectModel projectModel = javaModel.getProjectModel();
          String projectModelID = projectModel == null ? "" : projectModel.getId().toString();
          String packageName = javaModel.getPackageName() == null ? "" : javaModel.getPackageName();
  
          return projectModelID + "_" + packageName + "_" + javaModel.getFileName();
      }
docs: helping methods