Android - Why do I get a nullpointer exception (SQLite)

Question

I am trying to get an average from my database. I instantiate a cursorloader, and then call a method to return a cursor. (loadInBackGround). According to logcat this is where I get the nullpointer exception:

MyCursor = MyCursorLoader.loadInBackground();

This is where I create MyCursorLoader:

SQLiteCursorLoaderAverage MyCursorLoader = new SQLiteCursorLoaderAverage(getApplicationContext(),databaseHelper,null,null);

Heres my loadInBackground method:

public Cursor loadInBackground() {
      Cursor cursor=buildCursor();

      if (cursor!=null) {
        // Ensure the cursor window is filled
        cursor.getCount();
      }

      return(cursor);
    }

Heres my buildCursorMethod:

protected Cursor buildCursor()
    {
        return(db.getReadableDatabase().rawQuery(rawQuery, null));
    }

With the rawquery being (SELECT avg(numbers) FROM random)

I then get the average by doing this: MyCursor.getDouble(0);

My LogCat:

03-19 22:19:28.013: E/AndroidRuntime(1226): Caused by: java.lang.NullPointerException

03-19 22:19:28.013: E/AndroidRuntime(1226): tcom.example.helloworld.SQLiteCursorLoaderAverage.buildCursor(SQLiteCursorLoaderAverage.java:52)

03-19 22:19:28.013: E/AndroidRuntime(1226):     at com.example.helloworld.AbstractCursorLoader.loadInBackground(AbstractCursorLoader.java:21)

03-19 22:19:28.013: E/AndroidRuntime(1226):     at com.example.helloworld.AverageActivity.onCreate(AverageActivity.java:27)

I have no idea why I get the nullpointer exception. Thanks very much!

EDIT: Here is where my db gets set, it starts out as null.

public class SQLiteCursorLoaderAverage extends AbstractCursorLoader
{
SQLiteOpenHelper db=null;
String rawQuery=null;
String[] args=null;
Context context;

/**
 * Creates a fully-specified SQLiteCursorLoader. See
 * {@link SQLiteDatabase#rawQuery(SQLiteDatabase, String, String[])
 * SQLiteDatabase.rawQuery()} for documentation on the
 * meaning of the parameters. These will be passed as-is
 * to that call.
 */
public SQLiteCursorLoaderAverage(Context newContext, SQLiteOpenHelper db, Bundle searchParameters, String[] args)
{
    super(newContext);
    this.db=db;
    this.rawQuery=createQuery(searchParameters);
    this.args=args;
    context = newContext;
}

...

Answer 1

Your db is most likely null, maybe you can debug to find out the value of it.