How to store database instance in Android so that it doesnt become null?

Question

I have several database accesses in my Android Application and hence I have a created a class with a static variable, so that I can store a single instance of the database for the whole application. Quite often, this instance becomes null. Hence I have checks all over the place if at all the instance becomes null, which i added after getting the null pointer error and to reinitialize it in this way :

public class DatabaseInstance {
    public static StoreDbHelper db = null;

    public DatabaseInstance(Context c){
        db = new StoreDbHelper(c);
    }

    public static StoreDbHelper getCheckedDatabaseInstance(Context c){
        if(db == null){
            db = new StoreDbHelper(c);

        }
        return db;
    }
}

Even tough I do this, sometimes I still get db to be null. I have a feeling there is a better way to do this, I have now added db!=null checks all over, but is there a better way to reinitialize the variable or may be store it somewhere? My error is in the code below (before I added all the db !=null checks), it says

java.lang.NullPointerException: Attempt to invoke virtual method 'java.util.ArrayList com.example.ramapriyasridharan.localstore.StoreDbHelper.getAccelerometerTable()' on a null object reference 

This means that my variable db is null right? I located the error to the line in the code below:

if(taskParams.getTag().equals(SEND_SUMMARIZED_DATA)) {
                if(db==null){
                    db = DatabaseInstance.getCheckedDatabaseInstance(this);
                }
                final Runnable run_summarized = new Runnable() {
                    @Override
                    public void run() {
                        // ACCELEROMETER
                        if (db != null) {
                            //This is the error below!!!!!!!!!!!!!!!!!!
                            ArrayList<AccelerometerStoreReturn> p = db.getAccelerometerTable();
                            if (p != null && db != null) {

My question is, how should I keep one instance of the db object for the application, and why again does it repeatedly become null??

Note : StoredbHelper is a class that has the creation of my database and insertions deletions so on... as methods. As I mentioned before the error seems to come for the fact that db is null. This is the constructor below:

public StoreDbHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

This is my getAccelerometer table is it helps :

public ArrayList<AccelerometerStoreReturn> getAccelerometerTable(){
        String q = "SELECT * FROM "+TABLE_STORE_ACCELEROMETER;
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.rawQuery(q, null);
        ArrayList<AccelerometerStoreReturn> p = new ArrayList<AccelerometerStoreReturn>();
        while(cursor.moveToNext()){
            p.add(new AccelerometerStoreReturn(cursor.getLong(4),cursor.getInt(5),cursor.getFloat(1),cursor.getFloat(2),cursor.getFloat(3),cursor.getInt(6),cursor.getInt(0)));
        }
        cursor.close();
        Log.d("DB", "p.size ="+p.size());
        return p;
    }

And this is how I initialize db :

StoreDbHelper db = DatabaseInstance.getCheckedDatabaseInstance(this);

Thank you for your time!!

Answer 1

Helper Class :

public class OpenHelper {

    public static SQLiteOpenHelper singleton;
    static Context context;
    static SQLiteDatabase db;
    public static final String DATABASE_NAME = "Demo.sqlite";
    public static final int DATABASE_VERSION = 1;

    public static SQLiteDatabase getDataHelper(Context context_) {
        context = context_;
        try {
            // Copy your database here
        } catch (IOException e) {
        }

        if (singleton == null) {
            openDb();
        }
        if (db != null) {
            if (!db.isOpen()) {
                openDb();
            }
        } else {
            openDb();
        }
        return db;
    }

    static void openDb() {

        singleton = new SQLiteOpenHelper(context, DATABASE_NAME, null,
                DATABASE_VERSION) {

            @Override
            public void onUpgrade(SQLiteDatabase db, int oldVersion,
                    int newVersion) {
            }

            @Override
            public void onCreate(SQLiteDatabase db) {
            }
        };
        db = singleton.getWritableDatabase();
    }


}

To access in other class :

SQLiteDatabase db = OpenHelper.getDataHelper(context);

Answer 2

Try modifying your DataBase class as follows.

public class DatabaseInstance {
    private static StoreDbHelper db = null;

    private DatabaseInstance(Context c){
        db = new StoreDbHelper(c);
    }

    public static StoreDbHelper getCheckedDatabaseInstance(Context c){
        if(db == null){
            db = new StoreDbHelper(c);

        }
        return db;
    }
}

By making so , you can restrict other classes accessing the StoreDbHelper db instance and modifying it.

Assuming your StoreDbHelper should be extending super class like this.

public class StoreDbHelper extends SQLiteOpenHelper