所以我正在为我的班级申请一个用户提交表单并将数据记录在数据库中的应用程序。现在，我正在尝试实施一项检查，检查用户尝试选择的用户名是否存在于数据库中。这是我现在的代码：
//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();
运行此命令时，if 语句会出错。这是日志猫：
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)
以下是我的内容提供者的一些内容：
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)";