Hello

import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.TextField;
import javafx.stage.Stage;
import org.joda.time.DateTime;

import java.net.URL;

public class Main01 extends Application {
  private final int width = 600;
  private final int height = 480;
  public Button myButton;
  public TextField myTextField;

  private URL loadResource(String name) {
    return getClass().getClassLoader().getResource(name);
  }

  public void showDateTime(ActionEvent event) {
    myTextField.setText(new DateTime().toString("yyyy-MM-dd HH:mm:ss.SSS"));
  }

  @Override
  public void start(Stage primaryStage) {
    try {
      Parent root = FXMLLoader.load(loadResource("Main01.fxml"));
      Scene scene = new Scene(root, width, height);
      scene.getStylesheets().add(loadResource("Main01.css").toExternalForm());
      primaryStage.setTitle("Hello");
      primaryStage.setScene(scene);
      primaryStage.show();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

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

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.AnchorPane?>
<AnchorPane xmlns:fx="http://javafx.com/fxml/1"
            xmlns="http://javafx.com/javafx/8"
            fx:controller="Main01">
  <children>
    <Button fx:id="myButton" layoutX="60.0" layoutY="73.0"
            mnemonicParsing="false" onAction="#showDateTime" prefHeight="23.0"
            prefWidth="120.0" text="Show Date Time"/>
    <TextField fx:id="myTextField" layoutX="60.0" layoutY="155.0"/>
  </children>
</AnchorPane>