code: private JavaFileObject getOverviewPath() {
          for (String[] opt : options) {
              if (opt[0].equals("-overview")) {
                  if (env.fileManager instanceof StandardJavaFileManager) {
                      StandardJavaFileManager fm = (StandardJavaFileManager) env.fileManager;
                      return fm.getJavaFileObjects(opt[1]).iterator().next();
                  }
              }
          }
          return null;
      }
docs: Return the path of the overview file and null if it does not exist.
  @return the path of the overview file and null if it does not exist.