code: @Override
      @Deprecated
      public Set<String> keySet() {
          // TODO: Java 6 ResourceBundle has keySet() which calls handleKeySet()
          // and caches the results.
          // When we upgrade to Java 6, we still need to check for isTopLevelResource().
          // Keep the else branch as is. The if body should just return super.keySet().
          // Remove then-redundant caching of the keys.
          Set<String> keys = null;
          ICUResourceBundle icurb = null;
          if(isTopLevelResource() && this instanceof ICUResourceBundle) {
              // We do not cache the top-level keys in this base class so that
              // not every string/int/binary... resource has to have a keys cache field.
              icurb = (ICUResourceBundle)this;
              keys = icurb.getTopLevelKeySet();
          }
          if(keys == null) {
              if(isTopLevelResource()) {
                  TreeSet<String> newKeySet;
                  if(parent == null) {
                      newKeySet = new TreeSet<String>();
                  } else if(parent instanceof UResourceBundle) {
                      newKeySet = new TreeSet<String>(((UResourceBundle)parent).keySet());
                  } else {
                      // TODO: Java 6 ResourceBundle has keySet(); use it when we upgrade to Java 6
                      // and remove this else branch.
                      newKeySet = new TreeSet<String>();
                      Enumeration<String> parentKeys = parent.getKeys();
                      while(parentKeys.hasMoreElements()) {
                          newKeySet.add(parentKeys.nextElement());
                      }
                  }
                  newKeySet.addAll(handleKeySet());
                  keys = Collections.unmodifiableSet(newKeySet);
                  if(icurb != null) {
                      icurb.setTopLevelKeySet(keys);
                  }
              } else {
                  return handleKeySet();
              }
          }
          return keys;
      }
docs: Returns a Set of all keys contained in this ResourceBundle and its parent bundles.
  @return a Set of all keys contained in this ResourceBundle and its parent bundles,
  which is empty if this is not a bundle or a table resource
  @deprecated This API is ICU internal only.
  @hide draft / provisional / internal are hidden on Android