JAVAFX HBox not showing

Question

What I'm trying to do:
Make a base navigation HBox nav (also called HBox nav in the code), then use that HBox nav to make specialised navigation systems for each scene/page (sort of like a main class and then sub classes, without using classes).

Issue:
The HBox simply isn't showing. It was working initially, when I individually made a unique navigation HBox for each page, but when I tried to make 1 main navigation system, which gets customized for every page, it stopped showing.

//imports:
package com.example.phonecustomiser;
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import java.io.IOException;

//actual code:
public class HelloApplication extends Application {
    @Override
    public void start(Stage stage) throws IOException {

        //universal buttons
        Button playButton = new Button("PLAY");
        playButton.setPrefWidth(100);
        Button menuButton = new Button("MENU PAGE");
        menuButton.setPrefWidth(100);
        Button startButton = new Button( "START PAGE");
        menuButton.setPrefWidth(100);


        //universal navigation
        HBox nav = new HBox();                        // <------ HBOX NAV
        nav.setMaxHeight(100);
        nav.setStyle("-fx-background-color: white");
        nav.setPadding(new Insets(40, 40, 40, 40));
        nav.setSpacing(10);
        nav.getChildren().addAll(playButton, menuButton);


        //startPage
        StackPane startCanvas = new StackPane();
        HBox startNav = nav;                               // <--- SPECIALISED NAV
        startNav.setStyle("-fx-background-color: red");
        startCanvas.getChildren().addAll(startNav);        // <--- NOT SHOWING

        //menuPage
        StackPane menuCanvas = new StackPane();
        HBox menuNav = nav;
        menuNav.setStyle("-fx-background-color: green");
        menuCanvas.setAlignment(menuNav, Pos.BOTTOM_CENTER);
        menuCanvas.getChildren().addAll(menuNav);




        //scenes
        Scene startScene = new Scene(startCanvas, 600, 600);
        Scene menuScene = new Scene(menuCanvas, 600, 600);

        //button functions
        menuButton.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {
                stage.setScene(menuScene);
            }
        });



        stage.setScene(startScene);
        stage.show();
    }

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

Answer 1

I was thinking about these objects like they are primitive types. So: int monkey = 5; int cow = monkey;

cow++; this does not affect monkey

But objects do not work this way. A work around solution is (that I found at least), making a whole new class and putting my navigation system in there, then calling new instances of the class for every scene/page. The class I made:

package com.example.phonecustomiser;
import javafx.geometry.Insets;
import javafx.scene.control.Button;
import javafx.scene.layout.HBox;

public class Menu {
    Button playButton = new Button("PLAY");
    Button menuButton = new Button("MENU PAGE");
    Button startButton = new Button( "START PAGE");

    HBox nav = new HBox();

    public void setUp(){
        playButton.setPrefWidth(100);
        menuButton.setPrefWidth(100);
        menuButton.setPrefWidth(100);
        nav.setMaxHeight(100);
        nav.setStyle("-fx-background-color: green");
        nav.setPadding(new Insets(40, 40, 40, 40));
        nav.setSpacing(10);
        nav.getChildren().addAll(playButton, menuButton);
    }

}

Implementation of that class:

StackPane startCanvas = new StackPane();
        Menu startNav = new Menu();
        startNav.setUp();
        startNav.nav.setStyle("-fx-background-color: red");
        startCanvas.getChildren().addAll(startNav.nav);
        startCanvas.setAlignment(startNav.nav, Pos.BOTTOM_CENTER);