JavaFX Multiple Dates Picker

MultiDatePicker.java

package com.mmdemirbas.lab;

/**
 * Created by md on 5.04.2016.
 */

import javafx.application.Application;
import javafx.application.Platform;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.embed.swing.JFXPanel;
import javafx.geometry.Bounds;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.input.KeyCode;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Popup;
import javafx.stage.Stage;
import javafx.stage.Window;
import javafx.util.Callback;
import javafx.util.StringConverter;

import javax.swing.*;
import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class MultiDatePicker extends Application {

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

    private static void launchSwing() {
        SwingUtilities.invokeLater(() -> {
            JFrame         frame   = new JFrame("Swing and JavaFX");
            final JFXPanel fxPanel = new JFXPanel();
            frame.add(fxPanel);
            frame.setVisible(true);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            Platform.runLater(() -> fxPanel.setScene(createScene()));
        });
    }

    private static Scene createScene() {
        Button                    addButton     = new Button("+");
        Button                    removeButton  = new Button("-");
        ObservableList<LocalDate> selectedDates = FXCollections.observableArrayList();
        ListView<LocalDate>       dateList      = new ListView<>(selectedDates);
        String                    pattern       = "yyyy-MM-dd";
        DateTimeFormatter         dateFormatter = DateTimeFormatter.ofPattern(pattern);
        DatePicker                datePicker    = new DatePicker();
        datePicker.setShowWeekNumbers(true);
        datePicker.setOnAction(event -> System.out.println("Selected date: " + datePicker.getValue()));
        datePicker.setPromptText(pattern);
        datePicker.setConverter(new StringConverter<LocalDate>() {
            @Override
            public String toString(LocalDate date) {
                return (date == null) ? "" : dateFormatter.format(date);
            }

            @Override
            public LocalDate fromString(String string) {
                return ((string == null) || string.isEmpty()) ? null : LocalDate.parse(string, dateFormatter);
            }
        });
        datePicker.setOnAction(event -> selectedDates.add(datePicker.getValue()));

        datePicker.setDayCellFactory(new Callback<DatePicker, DateCell>() {
            @Override
            public DateCell call(DatePicker param) {
                return new DateCell() {
                    @Override
                    public void updateItem(LocalDate item, boolean empty) {
                        super.updateItem(item, empty);
                        boolean alreadySelected = selectedDates.contains(item);
                        setDisable(alreadySelected);
                        setStyle(alreadySelected ? "-fx-background-color: #09a30f;" : "");
                    }
                };
            }
        });

        // TODO: Hide text field of the date picker combo. Show dropdown directly on clicking "+" button.
        // TODO: Keep dropdown of the date picker combo open until intentionally clicking some other where.

        dateList.setOnKeyPressed(event -> {
            if (event.getCode() == KeyCode.DELETE) {
                removeSelectedDates(selectedDates, dateList);
            }
        });
        dateList.getSelectionModel().setSelectionMode(SelectionMode.MULTIPLE);
        removeButton.disableProperty().bind(dateList.getSelectionModel().selectedItemProperty().isNull());
        addButton.setOnAction(event -> {
            Popup popup = new Popup();
            popup.getContent().add(datePicker);
            popup.setAutoHide(true);
            Window window = addButton.getScene().getWindow();
            Bounds bounds = addButton.localToScene(addButton.getBoundsInLocal());
            double x      = window.getX() + bounds.getMinX();
            double y      = window.getY() + bounds.getMinY() + bounds.getHeight() + 5;
            popup.show(addButton, x, y);
            datePicker.show();
        });
        removeButton.setOnAction(event -> removeSelectedDates(selectedDates, dateList));

        HBox buttons = new HBox(addButton, removeButton);
        return new Scene(new VBox(buttons, dateList));
    }

    private static boolean removeSelectedDates(ObservableList<LocalDate> selectedDates, ListView<LocalDate> dateList) {
        return selectedDates.removeAll(dateList.getSelectionModel().getSelectedItems());
    }

    @Override
    public void start(Stage primaryStage) {
        primaryStage.setScene(createScene());
        primaryStage.show();
    }
}

This looks ideal for my JavaFX app. I changed the package and the class names. Unfortunately when I tried to use it, I hit a problem with JFXPanel.
Please can you help?

"C:\Program Files\Java\jdk-15.0.2\bin\java.exe" --module-path "C:\Program Files\Java\javafx-sdk-16\lib" --add-modules javafx.controls,javafx.fxml --add-exports javafx.graphics/com.sun.javafx.sg.prism=ALL-UNNAMED "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.3.2\lib\idea_rt.jar=50564:C:\Program Files\JetBrains\IntelliJ IDEA Community Edition 2020.3.2\bin" -Dfile.encoding=UTF-8 -classpath "C:\Users\Ted\IdeaProjects\PerfectWindows\out\production\Fixtures;C:\Program Files\Java\javafx-sdk-16\lib\src.zip;C:\Program Files\Java\javafx-sdk-16\lib\javafx-swt.jar;C:\Program Files\Java\javafx-sdk-16\lib\javafx.web.jar;C:\Program Files\Java\javafx-sdk-16\lib\javafx.base.jar;C:\Program Files\Java\javafx-sdk-16\lib\javafx.fxml.jar;C:\Program Files\Java\javafx-sdk-16\lib\javafx.media.jar;C:\Program Files\Java\javafx-sdk-16\lib\javafx.swing.jar;C:\Program Files\Java\javafx-sdk-16\lib\javafx.controls.jar;C:\Program Files\Java\javafx-sdk-16\lib\javafx.graphics.jar" fixturesPkg.Main
Exception in thread "AWT-EventQueue-0" java.lang.IllegalAccessError: class javafx.embed.swing.JFXPanel (in unnamed module @0x676e7187) cannot access class com.sun.javafx.logging.PlatformLogger (in module javafx.base) because module javafx.base does not export com.sun.javafx.logging to unnamed module @0x676e7187
at javafx.embed.swing.JFXPanel.(JFXPanel.java:136)
at fixturesPkg.Main.lambda$launchSwing$1(Main.java:38)
at java.desktop/java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:316)
at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:770)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721)
at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85)
at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:740)
at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124)
at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109)
at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

I was using JDK 8 when I wrote this piece of code. I can see from the output that you are using JDK 15. The error seems related to the Java module system which is introduced on JDK 9 as I remember. I'm afraid I don't know much about the module system. But I can say that your problem is unrelated to the MultiDatePicker code itself. I believe you can find some related material when you search for the error on the Internet. For example, I found the following quick results when I searched this phrase:

IllegalAccessError: class javafx.embed.swing.JFXPanel (in unnamed module @0x676e7187) cannot access class com.sun.javafx.logging.PlatformLogger (in module javafx.base) because module javafx.base does not export com.sun.javafx.logging to unnamed module

• https://stackoverflow.com/questions/57620350/how-to-fix-exception-when-creating-jfxpanel-in-swing-with-javafx-12
• javafxports/openjdk-jfx#548
• https://stackoverflow.com/questions/54291958/javafx-11-illegalaccesserror-when-creating-label/54292408#54292408
  I hope it helps.