Question

So I’m making an application for my class where the user submits a form and the data is recorded in a database. Right now I’m trying to implement a check that checks if the username the user is trying to pick exists in the database or not. Here’s my code right now:

//Username duplicate check                                                                                                                       
String mSelection = MyContentProvider.COLUMN_USERNAME + "= ?";
String[] mSelectionArgs = new String[] { "Username" };
String[] mProjection = new String[]{MyContentProvider.COLUMN_USERNAME};
mCursor = getContentResolver().query(MyContentProvider.CONTENT_URI,
                                     mProjection,
                                     mSelection,
                                     mSelectionArgs, 
                                     null);
if (mCursor.moveToFirst()){
if (userName.getText().toString().equals(mCursor.getString(mCursor.getColumnIndex(MyContentProvider.COLUMN_USERNAME)))){
    submitCheck = false;
    userName.setHint("Username taken");
}   

}
mCursor.close();

When this is run, the if statement garners an error. Here’s the logcat:

06-24 00:56:55.125: E/AndroidRuntime(907): FATAL EXCEPTION: main
06-24 00:56:55.125: E/AndroidRuntime(907): android.database.CursorIndexOutOfBoundsException: Index -1 requested, with a size of 0
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.database.AbstractCursor.checkPosition(AbstractCursor.java:400)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.database.AbstractWindowedCursor.checkPosition(AbstractWindowedCursor.java:136)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.database.AbstractWindowedCursor.getString(AbstractWindowedCursor.java:50)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.database.CursorWrapper.getString(CursorWrapper.java:114)
06-24 00:56:55.125: E/AndroidRuntime(907):  at com.connor.black.HomeWork2Activity$1.onClick(HomeWork2Activity.java:132)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.view.View.performClick(View.java:3511)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.view.View$PerformClick.run(View.java:14105)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.os.Handler.handleCallback(Handler.java:605)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.os.Handler.dispatchMessage(Handler.java:92)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.os.Looper.loop(Looper.java:137)
06-24 00:56:55.125: E/AndroidRuntime(907):  at android.app.ActivityThread.main(ActivityThread.java:4424)
06-24 00:56:55.125: E/AndroidRuntime(907):  at java.lang.reflect.Method.invokeNative(Native Method)
06-24 00:56:55.125: E/AndroidRuntime(907):  at java.lang.reflect.Method.invoke(Method.java:511)
06-24 00:56:55.125: E/AndroidRuntime(907):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
06-24 00:56:55.125: E/AndroidRuntime(907):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
06-24 00:56:55.125: E/AndroidRuntime(907):  at dalvik.system.NativeStart.main(Native Method)

Here’s a little bit of my content provider for some context:

public class MyContentProvider extends ContentProvider{
public final static String DBNAME = "FormStorage";
public static final String AUTHORITY = "com.connor.black.provider";
public final static String TABLE_NAMESTABLE = "formstable";
private static UriMatcher sUriMatcher;

public final static String COLUMN_FIRSTNAME = "FirstName";
public final static String COLUMN_LASTNAME = "LastName";
public final static String COLUMN_PHONE = "PhoneNumber";
public final static String COLUMN_EMAIL = "Email";
public final static String COLUMN_USERNAME = "Username";
public final static String COLUMN_PASSWORD = "Password";
public final static String COLUMN_GENDER = "Gender";
public final static String COLUMN_COUNTRY = "Country";
public static final Uri CONTENT_URI = Uri.parse("content://com.connor.black.provider/" + TABLE_NAMESTABLE);


private static final String SQL_CREATE_MAIN = "CREATE TABLE " + TABLE_NAMESTABLE + "("+"_ID INTEGER PRIMARY KEY, "+
                                                                    "FirstName TEXT, " +
                                                                    "LastName TEXT, " +
                                                                    "PhoneNumber TEXT, " +
                                                                    "Email TEXT, " +
                                                                    "Username TEXT, " +
                                                                    "Password TEXT, " +
                                                                    "Gender TEXT, " +
                                                                    "Country TEXT)";

Answer 1

You should not hard code the column index values directly into your code. This is because,

1. It’s poor design.

2. It makes your project difficult to maintain (i.e. if you add a column to your table later, it is possible that the column’s integer could change, thus breaking your code).

3. There is no way to know for certain if 0 corresponds to the username column. The integer value is assigned by an internal library that manages your SQLite database… you should never rely on internal libraries as the Android team could change the implementation at any time.

I suggest you change your code to the following:

int userCol = mCursor.getColumnIndex(MyContentProvider.USERNAME_COLUMN);
mCursor.getString(userCol);