Question

I have a database helper class (code below).
This helper’s class task is to copy the database from my assets folder shipped with the app into the data\data… of my app so that i can use it.

Once I get the database into data\data (which i am able to).. I want to add on it and perform the CRUD operations, and this database shall remain in the app untill user removes the app.

However, Once the copying is done and I have the database in data\data.. I try to create an instance of DatabaseHelper in a different activity (the one that i want to add data from)..
and when I do so,,, I am getting a null pointer exception error the moment the code reaches
DatabaseHelper dbh = new DatabaseHelper(this);
I know possible causes for this can be wrong context, or a null database… but all seem to be okay in my case.

Below are the codes.

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper
{   
    String DB_PATH = null;
// assign the database givens, such as name and context.
private static String DB_NAME = "offline";
private SQLiteDatabase myDataBase;
private final Context myContext;

static int count = 0;

/**
 * Constructor Takes and keeps a reference of the passed context in order to
 * access to the application assets and resources.
 * 
 * @param context
 */
public DatabaseHelper(Context context)
{
    super(context, DB_NAME, null, 1);
    this.myContext = context;
    DB_PATH = "/data/data/" + context.getPackageName() + "/" + "databases/";
}

// ///////////////////////////////////////////////////////////////////////////////////////

public void createDataBase() throws IOException
{
    boolean dbExist = checkDataBase();

    if (dbExist)
    {
        // do nothing - database already exist
    } else
    {
        this.getWritableDatabase();

        try
        {
            copyDataBase();

        } catch (IOException e)
        {
            throw new Error("Error copying database");
        }
    }
}

// ///////////////////////////////////////////////////////////////////////////////////////

private boolean checkDataBase()
{
    SQLiteDatabase checkDB = null;
    try
    {
        String myPath = DB_PATH + DB_NAME;
        checkDB = SQLiteDatabase.openDatabase(myPath, null,SQLiteDatabase.OPEN_READWRITE);
    } catch (SQLiteException e)
    {
        // database does't exist yet.
    }
    if (checkDB != null)
    {
        checkDB.close();
    }
    return checkDB != null ? true : false;
}

// ///////////////////////////////////////////////////////////////////////////////////////

private void copyDataBase() throws IOException
{
    // Open your local db as the input stream
    InputStream myInput = myContext.getAssets().open(DB_NAME);

    // Path to the just created empty db
    String outFileName = DB_PATH + DB_NAME;

    // Open the empty db as the output stream
    OutputStream myOutput = new FileOutputStream(outFileName);

    // transfer bytes from the inputfile to the outputfile
    byte[] buffer = new byte[1024];
    int length;
    while ((length = myInput.read(buffer)) > 0)
    {
        myOutput.write(buffer, 0, length);
    }

    // Close the streams
    myOutput.flush();
    myOutput.close();
    myInput.close();
}

// ///////////////////////////////////////////////////////////////////////////////////////

public void openDataBase() throws SQLException
{

    // Open the database
    String myPath = DB_PATH + DB_NAME;
    myDataBase = SQLiteDatabase.openDatabase(myPath, null,SQLiteDatabase.OPEN_READWRITE);
}

// ///////////////////////////////////////////////////////////////////////////////////////

@Override
public synchronized void close()
{
    if (myDataBase != null)
        myDataBase.close();

    super.close();
}

// return cursor
public Cursor query(String table, String[] columns, String selection,
        String[] selectionArgs, String groupBy, String having,
        String orderBy)
{
    return myDataBase.query("myTable", null, null, null, null, null, null);

}

// //////////////////////////////////////////////////////////////////////////////////

@Override
public void onCreate(SQLiteDatabase db)
{

}

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

}

// //////////////////////////////////////////////////////////////////////////////////
public long insertPuzzle(String q,String a,String c1,String c2,String c3,String c4)
{               
            ContentValues initialValues = new ContentValues();
                initialValues.put("q", q);
                initialValues.put("a",a);
                initialValues.put("c1",c1);
                initialValues.put("c2",c2);
                initialValues.put("c3",c3);
                initialValues.put("c4",c4);
                return myDataBase.insert("myTable", null, initialValues);

}

Whenever I do in my activity

DatabaseHelper db1 = new DatabaseHelper(this);
db1.openDatabase();
db1.insertPuzzle("q","a","1","2","3","4");

I get the null pointer error.

Hope you can be of help,
Regards,

Answer 1

put this file in your package

public class DBAdapter extends SQLiteOpenHelper {

private static String DB_PATH = "";
private static final String DB_NAME = "temp.sqlite";//your database name with extention any
private SQLiteDatabase myDataBase;
private final Context myContext;

private static DBAdapter mDBConnection;

/**
 * Constructor
 * Takes and keeps a reference of the passed context in order to access to the application assets and resources.
 * @param context
 */
private DBAdapter(Context context) {
    super(context, DB_NAME, null, 1);
    this.myContext = context;
    DB_PATH = "/data/data/"
            + context.getApplicationContext().getPackageName()
            + "/databases/";
    // The Android's default system path of your application database is
    // "/data/data/mypackagename/databases/"
}

/**
 * getting Instance
 * @param context
 * @return DBAdapter
 */
public static synchronized DBAdapter getDBAdapterInstance(Context context) {
    if (mDBConnection == null) {
        mDBConnection = new DBAdapter(context);
    }
    return mDBConnection;
}

/**
 * Creates an empty database on the system and rewrites it with your own database.
 **/
public void createDataBase() throws IOException {

    boolean dbExist = checkDataBase();

    if (dbExist) {
        // do nothing - database already exist
    } else {
        // By calling following method
        // 1) an empty database will be created into the default system path of your application
        // 2) than we overwrite that database with our database.
        this.getReadableDatabase();
        try {
            copyDataBase();
        } catch (IOException e) {
            throw new Error("Error copying database");
        }
    }
}

/**
 * Check if the database already exist to avoid re-copying the file each time you open the application.
 * @return true if it exists, false if it doesn't
 */
private boolean checkDataBase() {
    SQLiteDatabase checkDB = null;
    try {
        String myPath = DB_PATH + DB_NAME;
        checkDB = SQLiteDatabase.openDatabase(myPath, null,
                SQLiteDatabase.OPEN_READONLY);

    } catch (SQLiteException e) {
        // database does't exist yet.
    }
    if (checkDB != null) {
        checkDB.close();
    }
    return checkDB != null ? true : false;
}

/**
 * Copies your database from your local assets-folder to the just created
 * empty database in the system folder, from where it can be accessed and
 * handled. This is done by transfering bytestream.
 * */
private void copyDataBase() throws IOException {
        // Open your local db as the input stream
    InputStream myInput = myContext.getAssets().open(DB_NAME);
        // Path to the just created empty db
    String outFileName = DB_PATH + DB_NAME;
        // Open the empty db as the output stream
    OutputStream myOutput = new FileOutputStream(outFileName);
        // transfer bytes from the inputfile to the outputfile
    byte[] buffer = new byte[1024];
    int length;
    while ((length = myInput.read(buffer)) > 0) {
        myOutput.write(buffer, 0, length);
    }
        // Close the streams
    myOutput.flush();
    myOutput.close();
    myInput.close();
}

/**
 * Open the database
 * @throws SQLException
 */
public SQLiteDatabase openDataBase() throws SQLException {
    String myPath = DB_PATH + DB_NAME;
    myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);       
    return myDataBase;
}

/**
 * Close the database if exist
 */
@Override
public synchronized void close() {
    if (myDataBase != null)
        myDataBase.close();
    super.close();
}

/**
 * Call on creating data base for example for creating tables at run time
 */
@Override
public void onCreate(SQLiteDatabase db) {
}

/**
 * can used for drop tables then call onCreate(db) function to create tables again - upgrade
 */
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}

Activity onCreate method

private DBAdapter dba;
     @Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.profile);
              dba = DBAdapter.getDBAdapterInstance(this);
    try {
        dba.createDataBase();
    } catch (IOException e) {
        Log.e("log_tag", e.getLocalizedMessage());
    }
}

and for insert code

    SQLiteDatabase sqldb = dba.openDataBase();
    ContentValues cv = new ContentValues();
    cv.put("field", fieldvalue);
    sqldb.insert(table, nullColumnHack, cv);
    sqldb.close();
    dba.close();
    cv = null;