code: public static Set<String> getKeys(JSONObject jsonObject) {
          Set<String> keys = new TreeSet<String>();
          Iterator<?> iter = jsonObject.keys();
          while (iter.hasNext()) {
              keys.add((String) iter.next());
          }
          return keys;
      }
docs: Collects all keys in {@code jsonObject}.
  
  @param jsonObject the {@link JSONObject} to get the keys of
  @return the set of keys