Android - Random NullPointerException crash reports

Question

Sometimes I'm receiving crash reports from google caused by random NullPointerExceptions (see above). I tried to reproduce those errors but I'm not able to catch them.

Examples of NPE I get :

Caused by: java.lang.NullPointerException
at com.gamequiz.databasemanager.CategoryManager.getAllCategories(CategoryManager.java:28)

---

Caused by: java.lang.NullPointerException
at com.gamequiz.databasemanager.QuestionManager.getQuestionsFromLevel(QuestionManager.java:30)
at com.gamequiz.databasemanager.QuestionManager.getNumberOfQuestionAnsweredFromLevel(QuestionManager.java:148)

---

I though that my dbHelper variable is null sometimes but I can't figure out why.

Since I don't know how to resolve that, I post the all steps of my code :

First of all I initialize all the managers in the LaunchActivity :

@Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.layout_launch);

        initializeAllManagers();

       //some stuff

}

public void initializeAllManagers(){
        InitializeAllManagers.init(getApplicationContext());
    }

In my InitializeAllManagers class, I set all the managers I need for the lifecycle of the app :

public class InitializeAllManagers {

    public static void init(Context context){
        DatabaseManager.init(context);
        CategoryManager.init(DatabaseManager.getInstance().getHelper());
        //and others initializations
    }
}

DatabaseManager class (initialization of the dbManager and dbHelper) :

public class DatabaseManager {

    private static DatabaseManager instance;
    private DatabaseHelper helper;

    public static void init(Context ctx) {
        if (instance==null) {
            instance = new DatabaseManager(ctx);
        }
    }

    public static DatabaseManager getInstance() {
        return instance;
    }

    private DatabaseManager(Context ctx) {
        helper = new DatabaseHelper(ctx);
    }

    public DatabaseHelper getHelper() {
        return helper;
    }
}

Finally there is an example of one manager :

public class CategoryManager {

    private static DatabaseHelper dbHelper;

    public static void init(DatabaseHelper dbHelperInstance) {
        dbHelper = dbHelperInstance;
    }

    public static ArrayList <Category> getAllCategories(){
            ArrayList <Category> cList = null;
            try {
                cList = (ArrayList<Category>) dbHelper.getCategoryDao().queryForAll();
            } catch (SQLException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            return cList;
    }
}

So I suspect that my dbHelper variable is null sometimes. Does anyone have an idea about how I can solve this issue ?

EDIT :

NPE mainly refers to this lines :

cList = (ArrayList<Category>) dbHelper.getCategoryDao().queryForAll(); 
Dao <Question, Long> questionDao = dbHelper.getQuestionDao();

That's why I suspect that dbHelper is null sometimes, and apparently crashes occurs when the app is sleeping for a moment (see feedback above).

---

Feedback of one user :

So, mainly if I leave the app without exiting it, the app will often crash when I try to go back to it. Sometimes I just get sent back to the menu, but mostly just all the way out of the app and I have to restart it to continue.

Answer 1

I suspect that this happens when the app is minimized for a while and then get re-opened. Android can and will remove Objects from the heap if it feels like it needs the memory. Especially static variables can be removed. Check static variables for null before you access them.

For example in your DatabaseManager class, create the instance in the getInstance() method instead of in init().

public static DatabaseManager getInstance() {
    if(instance == null) instance = new DatabaseManager();
    return instance;
}

Edit:
Please note that my reasoning in this answer is wrong. Please read all the comments for clarification.