What you want is selection handler. If you want single slection use SingleSelectionHandler, if you want multiple selections use MultiSelectionHandler, Sample code :
SelectionModel<Item> selectionModel = new SingleSelectionModel<Item>();
cellList.setSelectionModel(selectionModel);
if you want to do anything on selection you can do it here
selectionModel.addSelectionChangeHandler(new SelectionChangeEvent.Handler() {
public void onSelectionChange(SelectionChangeEvent event) {
/** Do your thing here **/
selectionModel.getSelectedObject(); //for single Selection
selectionModel.getSelectedSet(); //for multiple Selection
}
});