code: public static String join(Collection<String> collection, String separator) {
          if (collection == null) {
              return null;
          }
          return join(collection.toArray(new String[collection.size()]), separator, 0, collection.size());
      }
docs: Join a collection of Strings
  
  @param collection strings to join
  @param separator  the separator to insert between the strings
  @return a string made of the strings in collection separated by separator