Question

Please help me, as I will go mad with this soon:

When I run the code, on first occasion loadNewPoint() is executed and displays some data from global variable – allPointsAndPlaces

However when I click a button (from a child class), the same method loadNewPoint() gives me null pointer for allPointsAndPlaces.

I have changed the code structure a lot from an original trying to solve this issue, and moved this method (loadNewPoint()) to a parent class to see, if it would solve the issue.

Parent class:

public class CabbieApp implements EntryPoint {

  private GetLocationsServiceAsync getAllLocationsService = GWT.create(GetLocationsService.class);
  CabbiePoint[] allPointsAndPlaces;
  PointsQuiz quiz;

  /**
   * Entry point method.
   */
  public void onModuleLoad() {

      //Get all the required data from DB
      getAllPointsAndLocations();

  }

  private void loadAppPages(){
      // Associate the Main panel with the HTML host page.
      RootPanel rootPanel = RootPanel.get("pointsList");    

      quiz = new PointsQuiz();

      rootPanel.setStyleName("GWTapp");
      rootPanel.add(quiz.getMainPanel());

      loadNewPoint();

  }


    private void getAllPointsAndLocations() {
        // Initialize the service proxy.
        if (getAllLocationsService == null) {
            getAllLocationsService = GWT.create(GetLocationsService.class);
        }

        // Set up the callback object.
        AsyncCallback<CabbiePoint[]> callback = new AsyncCallback<CabbiePoint[]>() {
          public void onFailure(Throwable caught) {
              System.out.println(caught.getMessage());
          }

          public void onSuccess(CabbiePoint[] result) {
            //allPointsAndPlaces = result;
            System.out.println(result.length);
            allPointsAndPlaces = result;
            loadAppPages();


          }
        };

        // Make the call to the service.
        getAllLocationsService.getLocations(callback);

    }


    void loadNewPoint(){
        int r = Random.nextInt(allPointsAndPlaces.length);
        quiz.CurrentPlace = allPointsAndPlaces[r].getPlaceName();
        quiz.CurrentLocation = allPointsAndPlaces[r].getPlaceLocation();

        quiz.point.setText(quiz.CurrentPlace);
        quiz.location.setText(quiz.CurrentLocation);
        quiz.location.setStyleName("invisibleText");
    }

  }

Child class:

public class PointsQuiz extends CabbieApp{

       VerticalPanel mainPanel = new VerticalPanel();
       HorizontalPanel navigation = new HorizontalPanel();
       TextBox point = new TextBox();
       TextBox location = new TextBox();
       Button showLocation = new Button("Show Location");
       Button nextPoint = new Button("Next Point");
       String CurrentPlace, CurrentLocation;


      public PointsQuiz() {

          // Assemble Add Stock panel.
              navigation.add(showLocation);
              navigation.add(nextPoint);
              navigation.setCellHorizontalAlignment(nextPoint, HasHorizontalAlignment.ALIGN_RIGHT);
              navigation.addStyleName("addPanel");
              mainPanel.setSpacing(5);
              mainPanel.setStyleName("body");
              mainPanel.setVerticalAlignment(HasVerticalAlignment.ALIGN_MIDDLE);
              mainPanel.setHorizontalAlignment(HasHorizontalAlignment.ALIGN_CENTER);

            // Assemble Main panel.
            mainPanel.add(point);
            point.setWidth("200px");
            mainPanel.add(location);
            location.setWidth("200px");
            mainPanel.add(navigation);
            navigation.setWidth("200px");

         // Move cursor focus to the input box.
            showLocation.setFocus(true);

         // Listen for mouse events on the show location button.
            showLocation.addClickHandler(new ClickHandler() {
              public void onClick(ClickEvent event) {
                showCurrentLocation();}
              });


             // Listen for mouse events on the next point button.
            nextPoint.addClickHandler(new ClickHandler() {
              public void onClick(ClickEvent event) {
                loadNewPoint();
                  }
            });       
      }



        private void showCurrentLocation(){
            location.setStyleName("visibleText");
        }

        public VerticalPanel getMainPanel() {
            return mainPanel;
        }


}

Answer 1

You are getting null pointer error because of allPointsAndPlaces is null. As per your coding The value of allPointsAndPlaces is assigned after completion of RPC call in getAllPointsAndLocations() method. so the allPointsAndPlaces has some assigned values.

Here you try to directly access loadNewPoint() method in child class. At a time, allPointsAndPlaces is not assigned.