code: public static String join(Collection collection, String separator) {
          if (isEmpty(collection)) {
              return StringUtils.EMPTY;
          }
          StringBuilder sb = new StringBuilder();
          for (Object object : collection) {
              if (object != null) {
                  String string = StringUtils.toString(object);
                  if (string != null) {
                      sb.append(string).append(separator);
                  }
              }
          }
          return sb.length() > 0 ? sb.substring(0, sb.length() - separator.length()) : StringUtils.EMPTY;
      }
docs: 连接集合类为字符串
  
  @param collection 集合
  @param separator  分隔符
  @return 分隔符连接的字符串