code: public void setSelectedIndex(int selectedIndex) {
          this.selectedIndex = selectedIndex;
          if (values.size() > 0) {
              T value = values.get(selectedIndex);
              if (value != null) {
                  $(listbox.getElement()).val(keyFactory.generateKey(value)).trigger("change.select2", selectedIndex);
              } else {
                  GWT.log("Value index is not found.", new IndexOutOfBoundsException());
              }
          }
      }
docs: Sets the currently selected index.
  <p>
  After calling this method, only the specified item in the list will
  remain selected. For a ListBox with multiple selection enabled.
  
  @param selectedIndex - the index of the item to be selected