code: public static String joinWithInnerSeparator(String separator,
                                                  Object... collection) {
          StringBuilder b = new StringBuilder();
          boolean first = true;
  
          for (Object o : collection) {
              if (first) {
                  first = false;
              } else {
                  b.append(separator);
              }
              b.append(o.toString());
              b.append(separator);
          }
          return b.toString();
      }
docs: Join an array of strings with a separator that appears after every
  instance in the list -except at the end
  
  @param collection strings
  @param separator  separator string
  @return the list