Have I used Singleton with Realm Database correct?

I have an app where the user can create/delete/edit to a list from a DialogFragment. Before I had a method like this in my DialogFragments: MainActivtity.adapter.add(String name, String location, double price);

adapter is my adapter object for the recyclerView. The class for the adapter had my create/delete/edit methods for the items in the recyclerView. Which was called like shown above which also is a horrible way to call mehtods as I understand.

So I choose to put all these CRUD methods in a one singleton class and call these method like this: Service.getInstance().add(...);

Is this a correct approach and what could I have done better?

This is how I made the singleton class that now contains my CRUD methods, instead of putting them in my adapter class for the recyclerView as before.

public class Service {

private static Realm realm;
private static Service service = new Service();

private Service() {
    realm = Realm.getInstance(App.getAppContex());
}

public static Service getInstance(){

    if(service == null){
        service = new Service();
    }
    return service;
}



   public void add(String name, String location, double price) {


    ShopListItem shopListItem = new ShopListItem();

    shopListItem.setName(name);
    shopListItem.setLocation(location);
    shopListItem.setPrice(price);
    shopListItem.setTimestamp(System.currentTimeMillis());
    shopListItem.setIsBought(0);

    realm.beginTransaction();
    realm.copyToRealm(shopListItem);
    realm.commitTransaction();
}


public void removeItem(int position, List<ShopListItem> shopListItems) {

    realm.beginTransaction();
    shopListItems.remove(position);
    realm.commitTransaction();
}

This class is just used for getting the global/Application context

public class App extends Application {


public static Application sApplication;

public static Application getsApplication(){
    return sApplication;
}

public static Context getAppContex(){

    return getsApplication();
}

@Override
public void onCreate() {
    super.onCreate();
    sApplication = this;
}
}