How in the JRuby..

Main.java

import javafx.application.Application;
import javafx.beans.binding.Bindings;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.control.TreeView;
import javafx.scene.layout.AnchorPane;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{

        MainItem i1 = new MainItem("Worlds");
        MainItem i2 = new MainItem("Planets");

        MainItem itemGroup = new MainItem("Hello");
        itemGroup.getChildren().addAll(i1, i2);

        TreeView allItems = new TreeView(itemGroup);

        Label someLabel = new Label();

        AnchorPane root = new AnchorPane();
        root.getChildren().addAll(allItems, someLabel);

        AnchorPane.setBottomAnchor(someLabel, 5.0);
        AnchorPane.setLeftAnchor(someLabel, 5.0);

        someLabel.textProperty().bind(Bindings.selectString(allItems.getSelectionModel().selectedItemProperty(), "plats"));


        primaryStage.setTitle("Hello Universe");
        primaryStage.setScene(new Scene(root));
        primaryStage.show();
    }


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

MainItem.java

import javafx.beans.property.SimpleStringProperty;
import javafx.beans.property.StringProperty;
import javafx.scene.control.TreeItem;

public class MainItem extends TreeItem {

    public StringProperty plats = new SimpleStringProperty();

    public String getPlats() {
        return plats.get();
    }

    public StringProperty platsProperty() {
        return plats;
    }

    public void setPlats(String appTitle) {
        this.plats.set(appTitle);
    }

    public MainItem(String x) {
        super(x);

        setPlats("oh look.. " + x);
    }
}