code: private ResourceBundle getBundle() {
          String bundleKey = getBundleKey();
          if (bundles.containsKey(bundleKey)) {
              return bundles.get(bundleKey);
          } else {
              ResourceBundle bundle = loadBundle();
              bundles.put(bundleKey, bundle);
              return bundle;
          }
      }
docs: Gets a bundle.  First tries to find one in the cache, then loads it if
  it can't find one.