TreeView one-at-a-time hander on CheckBoxTreeItem changes

I'm beginner in JavaFX.

I need to make one-at-a-time handler for any changes in the check boxes for my TreeView. Now it works like it handles all selected nodes simultaneously.

Thanks for help. Here is my code:

    package sample;

    import javafx.application.Application;
    import javafx.scene.Scene;
    import javafx.scene.control.CheckBoxTreeItem;
    import javafx.scene.control.TreeView;
    import javafx.scene.control.cell.CheckBoxTreeCell;
    import javafx.scene.layout.BorderPane;
    import javafx.stage.Stage;

    public class Main extends Application {

        @Override
        public void start(Stage primaryStage) throws Exception{
            BorderPane root = new BorderPane();
            root.setCenter(initTreeView());
            primaryStage.setTitle("Hello World");
            primaryStage.setScene(new Scene(root, 300, 275));

            primaryStage.show();
        }

        public TreeView initTreeView() {
            TreeView<String> treeView = new TreeView<>();
            treeView.setRoot(new CheckBoxTreeItem<>("123"));
            treeView.getRoot().addEventHandler(CheckBoxTreeItem.checkBoxSelectionChangedEvent(), event -> System.out.println("hello"));
            treeView.setCellFactory(p -> new CheckBoxTreeCell<>());
            treeView.getRoot().getChildren().addAll(new CheckBoxTreeItem<>("1"),new CheckBoxTreeItem<>("2"), new CheckBoxTreeItem<>("3"));
            treeView.getRoot().getChildren().get(0).getChildren().addAll(new CheckBoxTreeItem<>("4"),new CheckBoxTreeItem<>("5"), new CheckBoxTreeItem<>("6"));
            treeView.getRoot().getChildren().get(1).getChildren().addAll(new CheckBoxTreeItem<>("7"),new CheckBoxTreeItem<>("8"), new CheckBoxTreeItem<>("9"));
            treeView.getRoot().getChildren().get(2).getChildren().addAll(new CheckBoxTreeItem<>("10"),new CheckBoxTreeItem<>("11"), new CheckBoxTreeItem<>("12"));
            return treeView;
        }


        public static void main(String[] args) {
            launch(args);
        }

    }