code: public static void mergeStringList(
        final List<String> destinationList, final List<String> sourceList) {
      if (sourceList != null) {
        for (String item : sourceList) {
          if (!destinationList.contains(item)) {
            destinationList.add(item);
          }
        }
      }
    }
docs: Merge elements in Source List into the Destination List
  
  @param destinationList the list which the source elements will be merged into
  @param sourceList source List