code: public void setSingleValue(T value, boolean fireEvents) {
          int index = this.values.indexOf(value);
          if (index < 0 && value instanceof String) {
              index = getIndexByString((String) value);
          }
  
          if (index > -1) {
              List<T> before = getValue();
              setSelectedIndex(index);
  
              if (fireEvents) {
                  ValueChangeEvent.fireIfNotEqual(this, before, Collections.singletonList(value));
              }
          }
      }
docs: Set the selected value using a single item, generally used
  in single selection mode.