Thread and Progress bar Java FX

main.java

package sample;

import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.concurrent.Task;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.ProgressBar;
import javafx.scene.control.TextField;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.paint.Color;
import javafx.stage.Stage;


public class Main extends Application {
    int litros;
    double preciofinal;
    public Label precio=new Label();
    TextField txtlitros;
    //tu tarea hacer
    Task copyWorker;
    public static void main(String[] args) {
        Application.launch(args);
    }
    @Override
    public void start(Stage primaryStage) {
        primaryStage.setTitle("Gasolinera");
        Group root = new Group();
        Scene scene = new Scene(root, 330, 120, Color.WHITE);

        BorderPane mainPane = new BorderPane();
        root.getChildren().add(mainPane);

        final Label label = new Label("Gasolina cargada");
        precio.setText("$");
        final ProgressBar progressBar = new ProgressBar(0);
        final HBox h1 = new HBox();
        h1.setSpacing(5);
        h1.setAlignment(Pos.CENTER);
        h1.getChildren().addAll(precio);
        mainPane.setTop(h1);

        final HBox hb = new HBox();
        hb.setSpacing(5);
        hb.setAlignment(Pos.CENTER);
        hb.getChildren().addAll(label, progressBar);
        mainPane.setCenter(hb);
        final Label litroslbl= new Label("Litros de gasolina:");
         txtlitros= new TextField("");
        final Button startButton = new Button("Start");
        final Button cancelButton = new Button("Cancel");
        final HBox hb2 = new HBox();
        hb2.setSpacing(5);
        hb2.setAlignment(Pos.CENTER);
        hb2.getChildren().addAll(litroslbl,txtlitros,startButton, cancelButton);
        mainPane.setBottom(hb2);

        startButton.setOnAction(new EventHandler<ActionEvent>() {

            public void handle(ActionEvent event) {
                //limpias el diseño al empezar la acción del botón
                startButton.setDisable(true);
                progressBar.setProgress(0);
                cancelButton.setDisable(false);
                copyWorker = createWorker();

                progressBar.progressProperty().unbind();
                progressBar.progressProperty().bind(copyWorker.progressProperty());

                copyWorker.messageProperty().addListener(new ChangeListener<String>() {
                    public void changed(ObservableValue<? extends String> observable, String oldValue, String newValue) {
                        System.out.println(newValue);

                    }
                });

                new Thread(copyWorker).start();
                preciofinal=Double.parseDouble(txtlitros.getText())*20;
                precio.setText("$"+preciofinal);
            }
        });
        cancelButton.setOnAction(new EventHandler<ActionEvent>() {
            public void handle(ActionEvent event) {
                startButton.setDisable(false);
                cancelButton.setDisable(true);
                copyWorker.cancel(true);
                progressBar.progressProperty().unbind();
                progressBar.setProgress(0);
                System.out.println("cancelado");
            }
        });
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public Task createWorker() {
        return new Task() {
            @Override
            protected Object call() throws Exception {
                litros=Integer.parseInt(txtlitros.getText());


                    System.out.println("Litros:"+litros);
                for (int i = 0; i < (litros/2); i++) {
                    Thread.sleep(1000);
//                    updateMessage("2000 milliseconds");
                    updateProgress(i + 1, litros/2);
                    System.out.println(i);

                }

                return true;
            }
        };
    }
}