code: public String[] getSelectedValues() {
          List<WebElement> options = getElement().findElements(By.tagName("option"));
          List<String> selected = new ArrayList<String>();
          for (WebElement option : options) {
              if (option.isSelected()) {
                  selected.add(option.getAttribute("value"));
              }
          }
          return (String[]) selected.toArray(new String[selected.size()]);
      }
docs: Gets multiple selected values.
  
  @return All selected values.