code: public Object set(int index, Object element) {
          Object oldObject = items.set(index, element);
          if (hasChanged(oldObject, element)) {
              fireContentsChanged(index);
          }
          return oldObject;
      }
docs: Set the value of a list element at the specified index.
  @param index of element to set
  @param element New element value
  @return old element value