code: public Set<String> getExpandedRows() {
  		Set<String> keys = getComponentModel().expandedRows;
  		if (keys == null) {
  			return Collections.emptySet();
  		} else {
  			return Collections.unmodifiableSet(keys);
  		}
  	}
docs: Retrieve the row keys that are expanded.
  <p>
  A row key uniquely identifies each row and is determined by the {@link TreeItemModel}. Refer to
  {@link TreeItemModel#getItemId(List)}.
  </p>
  
  @return the expanded row keys.