code: public List<String> eachAttr(String attributeKey) {
          List<String> attrs = new ArrayList<>(size());
          for (Element element : this) {
              if (element.hasAttr(attributeKey))
                  attrs.add(element.attr(attributeKey));
          }
          return attrs;
      }
docs: Get the attribute value for each of the matched elements. If an element does not have this attribute, no value is
  included in the result set for that element.
  @param attributeKey the attribute name to return values for. You can add the {@code abs:} prefix to the key to
  get absolute URLs from relative URLs, e.g.: {@code doc.select("a").eachAttr("abs:href")} .
  @return a list of each element's attribute value for the attribute