Keep getting NullPointerExceptions when attempting to call getWritableDatabase()

Question

Hi all, I'm new to the Android framework and I can't get past first base with SQLite.

I'm trying to build a very simple application that has a EditText search box, which when a key is pressed performs a Like %word% search on a SQLite database for for the text entered in the EditText box, and displays the results in a ListView.

N.B. the below code included debugging and commented code, sorry I haven't got round to cleaning it up yet.

The Activate is

public class sustainable_fish extends Activity {

private String keycodeToAscii(int arg2){

    String retvar = "";

    switch(arg2){
    case KeyEvent.KEYCODE_A: retvar = "a";break;
    case KeyEvent.KEYCODE_B: retvar = "b";break;

snipped, but you get the idea.

    case KeyEvent.KEYCODE_RIGHT_BRACKET: retvar = ")";break;
    default: retvar = ""; break;
    };
    return retvar;

}



 Context that = getApplicationContext();
 fishDB dh = new fishDB(getApplicationContext());


private OnKeyListener search = new OnKeyListener() {

    public boolean onKey(View arg0, int arg1, KeyEvent arg2) {


        @SuppressWarnings("unused")
        Cursor cur = null;
        String searchData;
        EditText myEditText = (EditText) findViewById(R.id.fish_search);
        CharSequence edit_text_value = myEditText.getText();
        searchData = edit_text_value.toString();
        searchData = searchData + keycodeToAscii(arg2.getKeyCode() );
        Log.d("onKey", "searchData = "+searchData);
        /*
        Commented out because as the database doesn't work, so theirs no point just yet searching.

        cur = fish.search(searchData);
        if (cur != null)
        {
            String[] displayFields = new String[] {cur.getString(2),
                    cur.getString(1)};

            int[] displayViews = new int[] { R.id.fish_rank,
                    R.id.fish_name};

            SimpleCursorAdapter adapter = new SimpleCursorAdapter(that, 
                    R.layout.text_list, cur, 
                    displayFields, displayViews);

            ListView myList=(ListView)findViewById(android.R.id.list);

            myList.setAdapter(adapter);
        } 
        */

        return false;
    }

};

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

   setContentView(R.layout.main);
   EditText searchBar = (EditText) findViewById(R.id.fish_search);
   searchBar.setOnKeyListener(search); 

}

}

The SQLite database is handled by fishDB

  public class fishDB {

 public static final String DATABASE_NAME = "fish.db3";
 public static final int DATABASE_VERSION = 1;
 private SQLiteDatabase database;
 private Context myContext;
 private static boolean booFirstrun = false;

 fishDB(Context inContext){
     myContext = inContext;
     OpenHelper helper = new OpenHelper(myContext);
     Log.w("fishDB", "Called OpenHelper");

             // Here be the problem...

     database = helper.getWritableDatabase();

 }

 public boolean firstRun(){
    return booFirstrun;
 }


   private static class OpenHelper extends SQLiteOpenHelper {

       private static final String CREATE_TABLE_FEEDS = "create table feeds (feed_id integer primary key autoincrement, "
           + "title text not null, url text not null);";

          OpenHelper(Context context) {
             super(context, DATABASE_NAME, null, DATABASE_VERSION);
             Log.w("OpenHelper", "Called superconstructor");
            }

          @Override
          public void onCreate(SQLiteDatabase db) {
              try {
              Log.w("OpenHelper", "in onCreate");
              booFirstrun = true;
              Log.w("OpenHelper", "set booFirstrun");
              db.beginTransaction();
              Log.w("OpenHelper", "db.beginTransaction");
              try {
                  // Create tables and test data
                  db.execSQL(CREATE_TABLE_FEEDS);
                  Log.w("OpenHelper", "execSQL");
                  db.setTransactionSuccessful();
                  Log.w("OpenHelper", "setTransactionSuccessful");
              } catch (SQLException e) {
                  Log.e("Error creating tables and debug data", e.toString());
                  throw e;
              } finally {
                  db.endTransaction();
              }
              } catch (Exception e) {
                  Log.e("OpenHelper", e.toString() );
              } finally {
              Log.w("OpenHelper", "out of onCreate");
              }
          }

          @Override
          public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
             Log.w("OpenHelper", "Upgrading database, this will drop tables and recreate.");
             onCreate(db);
          }
       } 
 }

Now every time "database = helper.getWritableDatabase();" is called the app crashes with a NullPointerException. I've managed to trace the error to the onCreate method of OpenHelper and the "db.beginTransaction();" line.

What am I missing?

Answer 1

Actually you do not need to start transaction at onCreate method of the helper. Try removing everything from the method except db.execSQL(CREATE_TABLE_FEEDS);

UPD: ok.. I was able to repeat the problem. Move fishDb field initialization to the onCreate() method of activity after that error disappears. It looks like it is not good idea to start db initialization before the onCreate is invoked.

So instead of

fishDB fishDb = new fishDB(getApplicationContext());

do

fishDB fishDb;

onCreate(){
  super.onCreate();
  fishDb = new new fishDB(getApplicationContext());
  .... //init listener, query db etc..
}

Answer 2

The following code is derived from the original Android NotePad tutorial:

public class FishDb {

private static final String TAG = "FishDb";
private DatabaseHelper mDbHelper;
private SQLiteDatabase mDb;

private static final String CREATE_TABLE_FEEDS = "create table feeds (_id integer primary key autoincrement, "
       + "title text not null, url text not null);";

public static final String DATABASE_NAME = "fish.db3";
public static final String DATABASE_TABLE = "feeds";
private static final int DATABASE_VERSION = 1;

private final Context mCtx;

private static class DatabaseHelper extends SQLiteOpenHelper {

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

    @Override
    public void onCreate(SQLiteDatabase db) {

        db.execSQL(CREATE_TABLE_FEEDS);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                + newVersion + ", which will destroy all old data");
        db.execSQL("DROP TABLE IF EXISTS feeds");
        onCreate(db);
    }
}

public DbAdapter(Context ctx) {
    this.mCtx = ctx;
}

public DbAdapter open() throws SQLException {
    mDbHelper = new DatabaseHelper(mCtx);
    mDb = mDbHelper.getWritableDatabase();
    return this;
}

public void close() {
    mDbHelper.close();
}

// ... The helper methods
}

You would instantiate in your Activities onCreate() and possibly open() in onResume(), close() on onPause():

private FishDb mDbHelper;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mDbHelper = new FishDb(this);
}

@Override
public void onResume() {
    super.onResume();
    try{
        mDbHelper.open();
    } catch(Exception e){
        Log.e("onResume", e.toString());
    }
}

@Override
public void onPause() {
    super.onPause();
    try{
        mDbHelper.close();
    }  catch(Exception e){
        Log.e("onPause", e.toString());
    }
}

Hope that helps.

EDIT: Use "_id" instead of "feed_id" for primary key.