JavaFX TreeView item dynamic loading demo with FXML.

MyApplicationClass.java

import javafx.animation.*;
import javafx.application.Application;
import javafx.event.*;
import javafx.fxml.FXMLLoader;
import javafx.scene.*;
import javafx.scene.image.Image;
import javafx.scene.input.MouseEvent;
import javafx.stage.*;
import javafx.util.Duration;

/** Sample application to demonstrate programming an FXML interface. */ 
public class MyApplicationClass extends Application {
  @Override public void start(final Stage stage) throws Exception {
    // load the scene fxml UI.
    // grabs the UI scenegraph view from the loader.
    // grabs the UI controller for the view from the loader.
    final FXMLLoader loader = new FXMLLoader(getClass().getResource("myInterface.fxml"));
    final Parent root = (Parent) loader.load();
    final MyControllerClass controller = loader.<MyControllerClass>getController();
 
    // continuously refresh the TreeItems.
    // demonstrates using controller methods to manipulate the controlled UI.
    final Timeline timeline = new Timeline(
      new KeyFrame(
        Duration.seconds(3), 
        new TreeLoadingEventHandler(controller)
      )
    );
    timeline.setCycleCount(Timeline.INDEFINITE);
    timeline.play();

    // close the app if the user clicks on anywhere on the window.
    // just provides a simple way to kill the demo app.
    root.addEventFilter(MouseEvent.MOUSE_CLICKED, new EventHandler<MouseEvent>() {
      @Override public void handle(MouseEvent t) {
        stage.hide();
      }
    });
   
    // initialize the stage.
    stage.setScene(new Scene(root));
    stage.initStyle(StageStyle.TRANSPARENT);
    stage.getIcons().add(new Image(getClass().getResourceAsStream("myIcon.png")));
    stage.show();
  }

  /** small helper class for handling tree loading events. */
  private class TreeLoadingEventHandler implements EventHandler<ActionEvent> {
    private MyControllerClass controller;
    private int idx = 0;
    
    TreeLoadingEventHandler(MyControllerClass controller) {
      this.controller = controller;
    }
    
    @Override public void handle(ActionEvent t) {
      controller.loadTreeItems("Loaded " + idx, "Loaded " + (idx + 1), "Loaded " + (idx + 2));
      idx += 3;
    }
  }

  // main method is only for legacy support - java 8 won't call it for a javafx application.
  public static void main(String[] args) { launch(args); }
}

MyControllerClass.java

import javafx.fxml.FXML;
import javafx.scene.control.*;

/** Sample controller class. */ 
public class MyControllerClass {
  // the FXML annotation tells the loader to inject this variable before invoking initialize.
  @FXML private TreeView<String> locationTreeView;
 
  // the initialize method is automatically invoked by the FXMLLoader - it's magic
  public void initialize() {
    loadTreeItems("initial 1", "initial 2", "initial 3");
  }  

  // loads some strings into the tree in the application UI.
  public void loadTreeItems(String... rootItems) {
    TreeItem<String> root = new TreeItem<String>("Root Node");
    root.setExpanded(true);
    for (String itemString: rootItems) {
      root.getChildren().add(new TreeItem<String>(itemString));
    }

    locationTreeView.setRoot(root);
  }
}

myInterface.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<AnchorPane id="AnchorPane" maxHeight="-Infinity" maxWidth="-Infinity" minHeight="-Infinity" minWidth="-Infinity" xmlns:fx="http://javafx.com/fxml" fx:controller="test.MyControllerClass">
  <TreeView fx:id="locationTreeView" layoutX="0" layoutY="0" prefHeight="193.0" prefWidth="471.0" />
</AnchorPane>

Answer to the StackOverflow question: JavaFX - Wrapping FXML to Java Class Controller