Nullpointer Exception on return value from a DAO method

Question

I am using a Roomdatabase and I wish to search get a single object from the database when I give it's name. For that I wrote this Query in the DAO :

 @Query("SELECT * FROM kuh_table WHERE name = :kuhName ")
    Kuh findKuh(String kuhName);

I call it in the repository this way :

public Kuh findKuh(String kuhName){
        final Kuh[] kuh = new Kuh[1];
        new Thread(new Runnable() {
            volatile boolean running = true;
            @Override
            public void run() {
                if(running!= true) {
                    return;
                }
                kuh[0] =kuhDAO.findKuh(kuhName);
                running = false;
            }
        }).start();
        return kuh[0];
    }

then in my ViewModel this way :

public Kuh findKuh(String kuhName){ return repository.findKuh(kuhName);}

I then initialize my ViewModel in a fragment and try using the method by giving a String like this:

MarkerApiKt.setMarkerTapListener(mapView, (MarkerTapListener) (new MarkerTapListener() {
                public void onMarkerTap(@NotNull View view, int x, int y) {
                    Intrinsics.checkNotNullParameter(view, "view");
                    if (view instanceof MapMarker) {
                        MarkerCallout callout = new MarkerCallout(context);
                        callout.setTitle(((MapMarker) view).getName());
                        callout.setSubTitle("position: " + ((MapMarker) view).getX() + " , " + ((MapMarker) view).getY());
                        Kuh kuh = kuhViewModel.findKuh(((MapMarker) view).getName());
                        Toast.makeText(context, "this is "+ kuh.getName(), Toast.LENGTH_SHORT).show();

but somehow the istance of my object is always null since I end up with a nullpointer exception. Any idea what I may be doing wrong?

Answer 1

So, as @Olli said, the problem was that my thread in my Repository didn't finish its execution, which is why it returned a null object.

I just changed my code this way and now it works fine

public Kuh findKuh(String kuhName) throws InterruptedException {
        final Kuh[] kuh = new Kuh[1];
        Thread t1 = new Thread(new Runnable() {
            volatile boolean running = true;
            @Override
            public void run() {
                if(running!= true) {
                    return;
                }
                kuh[0] =kuhDAO.findKuh(kuhName);
                running = false;
            }
        });
        t1.start();
        t1.join();
        return kuh[0];
    }