MyMain.java

package com.shubhankar.JavaFxapp;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.fxml.FXMLLoader;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.layout.Pane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import java.util.Optional;

public class MyMain extends Application {

    public static void main(String[] args){
        System.out.println("main");
        launch(args);
    }

    @Override
    public void init() throws Exception {
        System.out.println("init");
        super.init();
    }

    @Override
    public void start(Stage primaryStage) throws Exception {

        System.out.println("Start");

        FXMLLoader loader = new
                FXMLLoader(getClass().getResource("app_layout.fxml"));
        VBox rootNode = loader.load();

        MenuBar menuBar = createMenu();
        rootNode.getChildren().add(0,menuBar);

        Scene scene = new Scene(rootNode);

        primaryStage.setScene(scene);
        primaryStage.setTitle("Temperature Converter Tool");
        primaryStage.show();
    }

    private MenuBar createMenu(){

        //file menu
        Menu fileMenu = new Menu("File");
        MenuItem newMenuItem = new MenuItem("New");
        newMenuItem.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {
                System.out.println("new menu item clicked");
            }
        });
        SeparatorMenuItem separatorMenuItem = new SeparatorMenuItem();
        MenuItem quitMenuItem = new MenuItem("Quit");
        quitMenuItem.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {
                Platform.exit();
                System.exit(0);
            }
        });
        fileMenu.getItems().addAll(newMenuItem,separatorMenuItem,quitMenuItem);
        //help menu
        Menu helpMenu = new Menu("Help");
        MenuItem aboutApp = new MenuItem("About");
        aboutApp.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent actionEvent) {
                aboutApp();
            }
        });
        helpMenu.getItems().addAll(aboutApp);

        //menu Bar
        MenuBar menuBar = new MenuBar();
        menuBar.getMenus().addAll(fileMenu,helpMenu);

        return menuBar;
    }

    private void aboutApp() {
        Alert alertDialog = new Alert(Alert.AlertType.INFORMATION);
        alertDialog.setTitle("My first desktop app");
        alertDialog.setHeaderText("Learning java");
        alertDialog.setContentText("i am just a beginner");

        ButtonType yesBtn = new ButtonType("Yes");
        ButtonType noBtn = new ButtonType("No");

        alertDialog.getButtonTypes().setAll(yesBtn,noBtn);

        Optional<ButtonType> clickedBtn =  alertDialog.showAndWait();
        if(clickedBtn.isPresent() && clickedBtn.get() == yesBtn){
            System.out.println("yes button clicked");
        }else {System.out.println("no button clicked");}
    }

    @Override
    public void stop() throws Exception {
        System.out.println("Stop");
        super.stop();
    }
}