Use el método setAlignment en JavaFX

import javafx.application.Application; // Package for the whole application
import javafx.geometry.Pos; // Geometric positioning Package for the variable "Pos"
import javafx.scene.Scene; // Package for the scene
import javafx.scene.control.Button; // Package for the button
import javafx.scene.layout.HBox; // The HBox Package
import javafx.stage.Stage; // Package for the stage

public class FxHBox extends Application { // Our main class extends to the Application class
  Button buttonOne, buttonTwo; // Declared two buttons
  HBox _hbox; // Declared a HBox
  Scene scene; // Declared a scene

  public void start(Stage BasicStage)
      throws Exception { // We have to use Exception Handling here as it can generate errors if
                         // anything goes wrong.
    BasicStage.setTitle("HBox Set Allignment"); // Set the  title of the application
    buttonOne = new Button("Button 1"); // Created button 1 with title
    buttonTwo = new Button("Button 2"); // Created button 2 with title

    _hbox = new HBox(buttonOne,
        buttonTwo); // Created an HBox element that holds the two buttons buttonOne & buttonTwo
    _hbox.setAlignment(
        Pos.CENTER); // Here we are customizing the default alignment and place our componet in
                     // center. Here 'Pos' provides the geometic allignment.

    scene = new Scene(_hbox, 400, 300); // Creates the scene with necessary heights and weights
    BasicStage.setScene(scene); // Set the scene
    BasicStage.show(); // Visualize the scene
  }

  public static void main(String[] args) {
    Application.launch(args); // Launch the application
  }
}

import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
import javafx.stage.Stage;

public class FxHBox extends Application {
  Button buttonOne, buttonTwo;
  HBox _hbox;
  Scene scene;

  public void start(Stage BasicStage) throws Exception {
    BasicStage.setTitle("HBox Set Allignment");
    buttonOne = new Button("Button 1");
    buttonTwo = new Button("Button 2");

    _hbox = new HBox(buttonOne, buttonTwo);
    _hbox.setAlignment(Pos.CENTER);

    scene = new Scene(_hbox, 400, 300);
    BasicStage.setScene(scene);
    BasicStage.show();
  }

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