Java Wie Variablen einer JavaFX-GUI an andere (aufrufende) Klassen übergeben?

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

<?import javafx.geometry.*?>
<?import java.lang.*?>
<?import java.util.*?>
<?import javafx.scene.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>

<AnchorPane id="AnchorPane" prefHeight="150.0" prefWidth="300.0" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8" fx:controller="stundenverwaltung.LogInDialogController">
   <children>
      <GridPane layoutX="20.0" layoutY="38.0" prefHeight="400.0" prefWidth="600.0" AnchorPane.bottomAnchor="0.0" AnchorPane.leftAnchor="0.0" AnchorPane.rightAnchor="0.0" AnchorPane.topAnchor="0.0">
        <columnConstraints>
          <ColumnConstraints hgrow="SOMETIMES" maxWidth="145.0" minWidth="10.0" prefWidth="108.0" />
          <ColumnConstraints hgrow="SOMETIMES" maxWidth="229.0" minWidth="10.0" prefWidth="192.0" />
        </columnConstraints>
        <rowConstraints>
          <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
          <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
          <RowConstraints minHeight="10.0" prefHeight="30.0" vgrow="SOMETIMES" />
        </rowConstraints>
         <children>
            <Label text="Nutzername:">
               <padding>
                  <Insets left="10.0" />
               </padding>
            </Label>
            <Label text="Passwort:" GridPane.rowIndex="1">
               <padding>
                  <Insets left="10.0" />
               </padding>
            </Label>
            <TextField fx:id="usernameId" promptText="Bitte Nutzername eingeben" GridPane.columnIndex="1">
               <opaqueInsets>
                  <Insets />
               </opaqueInsets>
               <GridPane.margin>
                  <Insets right="10.0" />
               </GridPane.margin>
            </TextField>
            <PasswordField fx:id="passwordId" onAction="#firePassword" promptText="Bitte Passwort eingeben" GridPane.columnIndex="1" GridPane.rowIndex="1">
               <opaqueInsets>
                  <Insets />
               </opaqueInsets>
               <GridPane.margin>
                  <Insets right="10.0" />
               </GridPane.margin>
            </PasswordField>
            <Button fx:id="buttonOkId" minWidth="70.0" mnemonicParsing="false" onAction="#fireOk" onMouseClicked="#fireOk" text="OK" GridPane.columnIndex="1" GridPane.halignment="LEFT" GridPane.rowIndex="2">
               <GridPane.margin>
                  <Insets left="10.0" />
               </GridPane.margin>
            </Button>
            <Button fx:id="buttonCancelId" minWidth="70.0" mnemonicParsing="false" onAction="#fireCancel" onMouseClicked="#fireCancel" text="Abbruch" GridPane.columnIndex="1" GridPane.halignment="RIGHT" GridPane.rowIndex="2">
               <GridPane.margin>
                  <Insets right="10.0" />
               </GridPane.margin>
            </Button>
         </children>
      </GridPane>
   </children>
</AnchorPane>

public class LogInDialogController implements Initializable {
    @FXML
    private TextField usernameId;
    @FXML
    private PasswordField passwordId;
    @FXML
    private Button buttonOkId;
    @FXML
    private Button buttonCancelId;
    private String user, password;
    /**
     * Initializes the controller class.
     */
    @Override
    public void initialize(URL url, ResourceBundle rb) {
        // TODO
        // Frage: Was tut diese Methode??
    }
    public void firePassword() {
        System.out.printf("Enter nach Passwort  %s %s", usernameId.getText(), passwordId.getText());
        close();
    }
    public void fireOk() {
        System.out.printf("Ok-Button:  %s %s", usernameId.getText(), passwordId.getText());
        setUser(getUsernameId().getText());
        setPassword(getPasswordId().getText());
System.out.printf("Ok-Button 2:  %s %s", getUser(), getPassword()); // später ergänzt
close();
    }
    public void fireCancel() {
        System.out.println("Abbruch-Button");
        close();
    }
    public void close() {
        Stage stage = (Stage) buttonCancelId.getScene().getWindow();
        stage.close();
    }
public TextField getUsernameId() {
        return usernameId;
    }
    public PasswordField getPasswordId() {
        return passwordId;
    }
public String getUser() {
        return user;
    }
    public void setUser(String user) {
        this.user = user;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
}

public class Login extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {
        primaryStage.setTitle("Login");
        Pane myPane = (Pane)FXMLLoader
                .load(getClass().getResource("LogInDialog.fxml"));
        Scene myScene = new Scene(myPane);
        primaryStage.setScene(myScene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
LogInDialogController dialog = new LogInDialogController();
        System.out.printf("User: %s, Passwort: %s", dialog.getUser(), dialog.getPassword());
    }
}

public class TestToDelete extends Stage {

    private String user = "", password = "";

    public TestToDelete() {
        super();
        setTitle("Login");
        initModality(Modality.APPLICATION_MODAL);
        
        Label l1 = new Label("Bitte Nutzername eingeben:");
        Label l2 = new Label("Bitte Passwort eingeben:");
        TextField tf = new TextField();
        PasswordField pf = new PasswordField();
        Button btnOk = new Button("OK");
        Button btnCancel = new Button("Abbruch");
        GridPane pane = new GridPane();
        pane.setAlignment(Pos.CENTER);
        pane.add(l1, 1, 1);
        pane.add(tf, 2, 1);
        pane.add(l2, 1, 2);
        pane.add(pf, 2, 2);
        pane.add(btnOk, 1, 3);
        pane.add(btnCancel, 2, 3);  
        btnOk.setOnAction(e -> {
            user = tf.getText();
            password = pf.getText();
            close();
        });

        btnCancel.setOnAction(e -> {
            close();
        });
        Scene scene = new Scene(pane, 400, 150);
        setScene(scene);
    }
	
    public String getUser() {
        return user;
    }

    public String getPassword() {
        return password;
    }
}

public class TestToDeleteStart {
    public static void main(String[] args) {
        TestToDelete ttd = new TestToDelete();
        ttd.showAndWait();
        System.out.printf("User: %s, Password: %s", ttd.getUser(), ttd.getPassword());
    }
}