Demo of a MVC-like approach to managing view switching in JavaFX. Contains three views: a login screen, and two other pages. The model holds a user and a "browse state". A view manager determines the current view based on values in the model. The FXML controllers/presenters have a reference to a shared model whose properties they update.

Book.fxml

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

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.VBox?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.control.Button?>
<VBox xmlns="http://javafx.com/javafx"
      xmlns:fx="http://javafx.com/fxml"
      spacing="20"
      alignment="CENTER"
      fx:controller="org.jamesd.examples.library.BookController">
    <padding><Insets topRightBottomLeft="20"/></padding>

    <Label text="Books" style="-fx-font: bold 24pt sans-serif;"/>
    <Label fx:id="welcomeLabel"/>
    <Label text="Please browse our books..."/>
    <HBox spacing="10" alignment="CENTER">
        <Button text="Switch to DVDs" onAction="#switchToDvds"/>
        <Button text="Logout" onAction="#logout"/>
    </HBox>
</VBox>

BookController.java

package org.jamesd.examples.library;

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

public class BookController {

    private final Model model;
    @FXML
    private Label welcomeLabel;

    public BookController(Model model) {
        this.model = model;
    }

    @FXML
    protected void initialize() {
        welcomeLabel.textProperty().bind(
                model.userProperty()
                        .map( u -> String.format("Welcome %s", u.username()))
                        .orElse("Welcome")
        );
    }

    @FXML
    private void switchToDvds() {
        model.setBrowseState(BrowseState.DVD);
    }

    @FXML
    private void logout() {
        model.logout();
    }
}

BrowseState.java

package org.jamesd.examples.library;
public enum BrowseState { BOOK, DVD }

Dvd.fxml

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

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.layout.VBox?>
<VBox xmlns="http://javafx.com/javafx"
      xmlns:fx="http://javafx.com/fxml"
      spacing="20"
      alignment="CENTER"
      fx:controller="org.jamesd.examples.library.DvdController">
    <padding><Insets topRightBottomLeft="20"/></padding>

    <Label text="DVDs" style="-fx-font: bold 24pt sans-serif;"/>

    <Label fx:id="welcomeLabel"/>
    <Label text="Please browse our DVDs..."/>
    <HBox spacing="10" alignment="CENTER">
        <Button text="Switch to Books" onAction="#switchToBooks"/>
        <Button text="Logout" onAction="#logout"/>
    </HBox>
</VBox>

DvdController.java

package org.jamesd.examples.library;

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

public class DvdController {

    private final Model model ;

    @FXML
    private Label welcomeLabel;

    public DvdController(Model model) {
        this.model = model;
    }

    @FXML
    protected void initialize() {
        welcomeLabel.textProperty().bind(
                model.userProperty()
                        .map( u -> String.format("Welcome %s", u.username()))
                        .orElse("Welcome")
        );
    }

    @FXML
    private void switchToBooks() {
        model.setBrowseState(BrowseState.BOOK);
    }

    @FXML
    private void logout() {
        model.logout();
    }
}

LibraryApp.java

package org.jamesd.examples.library;

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.stage.Stage;

public class HelloApplication extends Application {
    @Override
    public void start(Stage stage) {
        Model model = new Model();
        ViewManager viewManager = new ViewManager(model);
        Scene scene = new Scene(viewManager.getCurrentView(), 800, 500);
        scene.rootProperty().bind(viewManager.currentViewProperty());
        stage.setScene(scene);
        stage.show();
    }

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

Login.fxml

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

<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.HBox?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.PasswordField?>
<?import javafx.scene.control.Tooltip?>
<GridPane xmlns="http://javafx.com/javafx"
          xmlns:fx="http://javafx.com/fxml"
          hgap="5" vgap="5"
          alignment="CENTER"
          fx:controller="org.jamesd.examples.library.LoginController">

    <Label fx:id="loginLabel" text="Please enter user name and password"
           GridPane.rowIndex="0" GridPane.columnIndex="0" GridPane.columnSpan="2"/>
    <Label text="Username:" GridPane.rowIndex="1" GridPane.columnIndex="0"/>
    <Label text="Password:" GridPane.rowIndex="2" GridPane.columnIndex="0"/>

    <TextField fx:id="usernameEntry" GridPane.rowIndex="1" GridPane.columnIndex="1" onAction="#login" />
    <PasswordField fx:id="passwordEntry" GridPane.rowIndex="2" GridPane.columnIndex="1"
            promptText="Password is 'secret'" onAction="#login" >
            <tooltip>
                <Tooltip text="Password is 'secret'"/>
            </tooltip>
    </PasswordField>

    <HBox alignment="CENTER" GridPane.rowIndex="3" GridPane.columnIndex="0" GridPane.columnSpan="2">
        <Button text="Login" onAction="#login" />
    </HBox>

</GridPane>

LoginController.java

package org.jamesd.examples.library;

import javafx.fxml.FXML;
import javafx.scene.control.Label;
import javafx.scene.control.PasswordField;
import javafx.scene.control.TextField;


public class LoginController {

    private final Model model;
    @FXML
    private Label loginLabel;
    @FXML
    private TextField usernameEntry;
    @FXML
    private PasswordField passwordEntry;

    public LoginController(Model model) {
        this.model = model;
    }


    public void login() {
        String username = usernameEntry.getText();
        String password = passwordEntry.getText();
        if (username.isBlank() || password.isBlank()) {
            loginLabel.setText("Please enter username and password");
        } else {
            if (validateLogin(username, password)) {
                model.setUser(new User(username));
            } else {
                loginLabel.setText("Invalid username and/or password");
            }
        }
    }

    public boolean validateLogin(String username, String password) {
        // dummy implementation
        return "secret".equals(password);
    }

}

Model.java

package org.jamesd.examples.library;

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;

public class Model {
    private final ObjectProperty<User> user = new SimpleObjectProperty<>(null);
    public User getUser() {
        return user.get();
    }
    public ObjectProperty<User> userProperty() {
        return user;
    }
    public void setUser(User user) {
        this.user.set(user);
    }

    private final ObjectProperty<BrowseState> browseState = new SimpleObjectProperty<>(BrowseState.BOOK);
    public BrowseState getBrowseState() {
        return browseState.get();
    }
    public ObjectProperty<BrowseState> browseStateProperty() {
        return browseState;
    }
    public void setBrowseState(BrowseState browseState) {
        this.browseState.set(browseState);
    }

    public void logout() {
        setUser(null);
    }
}

module-info.java

module org.jamesd.examples.library {
    requires javafx.controls;
    requires javafx.fxml;

    opens org.jamesd.examples.library to javafx.fxml;
    exports org.jamesd.examples.library;
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.jamesd.examples</groupId>
    <artifactId>library</artifactId>
    <version>1.0-SNAPSHOT</version>
    <name>library</name>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <java.version>20</java.version>
        <javafx.version>20</javafx.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-controls</artifactId>
            <version>${javafx.version}</version>
        </dependency>
        <dependency>
            <groupId>org.openjfx</groupId>
            <artifactId>javafx-fxml</artifactId>
            <version>${javafx.version}</version>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.10.1</version>
                <configuration>
                    <source>${java.version}</source>
                    <target>${java.version}</target>
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.openjfx</groupId>
                <artifactId>javafx-maven-plugin</artifactId>
                <version>0.0.8</version>
                <executions>
                    <execution>
                        <!-- Default configuration for running with: mvn clean javafx:run -->
                        <id>default-cli</id>
                        <configuration>
                            <mainClass>org.jamesd.examples.library/org.jamesd.examples.library.HelloApplication
                            </mainClass>
                            <launcher>app</launcher>
                            <jlinkZipName>app</jlinkZipName>
                            <jlinkImageName>app</jlinkImageName>
                            <noManPages>true</noManPages>
                            <stripDebug>true</stripDebug>
                            <noHeaderFiles>true</noHeaderFiles>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

User.java

package org.jamesd.examples.library;
public record User(String username) {}

ViewManager.java

package org.jamesd.examples.library;

import javafx.beans.property.ReadOnlyObjectProperty;
import javafx.beans.property.ReadOnlyObjectWrapper;
import javafx.beans.value.ChangeListener;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.util.Callback;

import java.io.IOException;
import java.lang.reflect.Constructor;
import java.net.URL;

public class ViewManager {

    private final Model model ;
    private final Callback<Class<?>, Object> controllerFactory ;
    
    private final ReadOnlyObjectWrapper<Parent> currentView = new ReadOnlyObjectWrapper<>();
    public Parent getCurrentView() {
        return currentView.get();
    }
    public ReadOnlyObjectProperty<Parent> currentViewProperty() {
        return currentView.getReadOnlyProperty();
    }

    public ViewManager(Model model) {
        this.model = model;
        
        // The controller factory is a functional interface that maps Class<?> instances
        // to objects. It is used by the FXMLLoader to create controllers for a given
        // controller class (specified by fx:controller in the FXML file).
        // Since our controllers don't have default constructors, we need a controller
        // factory to determine how to instantiate the controllers. 
        // The implementation here looks for a constructor with a single parameter of type Model,
        // and if it finds one invokes that constructor, providing the model instance.
        controllerFactory = type -> {
            try {
                for (Constructor<?> c : type.getConstructors()) {
                    if (c.getParameterCount() == 1 && c.getParameterTypes()[0].equals(Model.class)) {
                        return c.newInstance(model);
                    }
                }
                return type.getConstructor().newInstance();
            } catch (Exception e) {
                throw e instanceof RuntimeException re ? re : new RuntimeException(e);
            }
        };

        // update the current view if either the user or the browse state change:
        ChangeListener<Object> listener = (obs, oldValue, newValue) -> updateView();
        model.userProperty().addListener(listener);
        model.browseStateProperty().addListener(listener);
        
        // And initialize the current view based on the current state of the model:
        updateView();
    }

    private void updateView() {
        try {
            // The FXML file to load:
            URL resource;
            // If no-one is logged in, provide the login page
            if (model.getUser() == null) {
                resource = LoginController.class.getResource("Login.fxml");
            } else {
                // Otherwise, provide the page determined by the "browse state":
                resource = switch(model.getBrowseState()) {
                    case DVD -> DvdController.class.getResource("Dvd.fxml");
                    case BOOK -> BookController.class.getResource("Book.fxml");
                };
            }
            FXMLLoader loader = new FXMLLoader(resource);
            loader.setControllerFactory(controllerFactory);
            currentView.set(loader.load());
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}