SQLite 游标返回 IllegalStateException？

Question

我正在尝试将元素添加到数据库，然后搜索特定条目。但是我得到一个游标错误。这是我的数据库代码 class。数据库只包含一个列。提前致谢。

public class DataB extends SQLiteOpenHelper {
    private static final String db_name = "testing.db";
    private static final int version = 1;
    private static final String table_name = "students";
    private static final String col_name="FirstName";
    public DataB(Context context) {
        super(context, db_name, null, version);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String q = "CREATE TABLE " + table_name + " (FirstName TEXT PRIMARY KEY) ";
        db.execSQL(q);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS "+table_name);
        onCreate(db);
    }
    public int addData(String name)
    {
        ContentValues cv = new ContentValues();
        cv.put(col_name, name);
        SQLiteDatabase db = this.getWritableDatabase();
        db.insert(table_name, null, cv);
        db.close();
        return 1;
    }
    public String search(String string)
    {
        String dbstring=" ";
        SQLiteDatabase db = this.getWritableDatabase();
        String query = "SELECT * FROM "+ table_name + " WHERE FirstName = '" + string+"'";
        Cursor c = db.rawQuery(query,null);
        c.moveToFirst();
        while(!c.isAfterLast())
        {
            if(c.getString(c.getColumnIndex("FirstName"))!=null)
            {
                dbstring = c.getString(c.getColumnIndex("FirstName"));
            }
        }
        return dbstring;
    }
}

错误是 Caused by: java.lang.IllegalStateException: Couldn\'t read row 0, col -1 from CursorWindow. Make sure the Cursor is initialized correctly before accessing data from it.

Answer 1

while 根本不工作，你永远不会改变光标位置，所以它总是正确的，所以制作一个 Whosebug

执行查询后，您应该询问光标是否不为空以及是否可以移动到第一个位置，如下所示：

    if(c != null && c.moveToFirst()){      
       dbstring = c.getString(c.getColumnIndex(col_name));           
     }

或者在您的情况下，因为您拥有的唯一列是 FirstName，您可以这样看：

    if(c != null && c.moveToFirst()){      
       dbstring = c.getString(0);           
    }

请记住，如果您更改了数据库中的任何内容，您应该升级数据库版本。

SQLite 游标返回 IllegalStateException？

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