LoginCode

// Create the custom dialog.
        Dialog<Pair<String, String>> dialog = new Dialog<>();
        Stage s = (Stage) dialog.getDialogPane().getScene().getWindow();
        s.getIcons().add(new Image(this.getClass().getResource("login.png").toString()));
        dialog.setTitle("Authentification");
        dialog.setHeaderText("\tVeuillez saisir vos identifiants \n  pour vous connecter à l'application\n\t\t\t\t↓");

        // Set the icon (must be included in the project).
        dialog.setGraphic(new ImageView(this.getClass().getResource("login.png").toString()));

        // Set the button types.
        ButtonType loginButtonType = new ButtonType("Connexion", ButtonData.OK_DONE);
        dialog.getDialogPane().getButtonTypes().addAll(loginButtonType, ButtonType.CANCEL);

        // Create the username and password labels and fields.
        GridPane grid = new GridPane();
        grid.setHgap(10);
        grid.setVgap(10);
        grid.setPadding(new Insets(20, 150, 10, 10));


        txtUser.setPromptText("Nom d'utilisateur");
        txtPass.setPromptText("Mot de passe");

        grid.add(new Label("Nom d'utilisateur:"), 0, 0);
        grid.add(txtUser, 1, 0);
        grid.add(new Label("Mot de passe:"), 0, 1);
        grid.add(txtPass, 1, 1);

        // Enable/Disable login button depending on whether a username was
        // entered.
        Node loginButton = dialog.getDialogPane().lookupButton(loginButtonType);
        loginButton.setDisable(true);

        // Do some validation (using the Java 8 lambda syntax).
        txtUser.textProperty().addListener((observable, oldValue, newValue) -> {
            loginButton.setDisable(newValue.trim().isEmpty());
        });

        dialog.getDialogPane().setContent(grid);

        // Request focus on the username field by default.
        Platform.runLater(() -> txtUser.requestFocus());

        // Convert the result to a username-password-pair when the login button
        // is clicked.
        dialog.setResultConverter(dialogButton -> {
            if (dialogButton == loginButtonType) {
                return new Pair<>(txtUser.getText(), txtPass.getText());
            }
            return null;
        });

        Optional<Pair<String, String>> result = dialog.showAndWait();

        result.ifPresent(usernamePassword -> {
            onConnect();
            System.out.println("Username=" + usernamePassword.getKey() + ", Password=" + usernamePassword.getValue());
        });