code: public static String getString(JSONObject json, String key) {
          return getString(json, key, null);
      }
docs: Returns the value of a JSON property as a String if it exists otherwise returns null.
  Same as calling {@link #getString(net.sf.json.JSONObject, java.lang.String, java.lang.String) }
  with null as defaultValue.
  @param json the JSONObject to check.
  @param key the key.
  @return the value for the key as a string.