SQLite游标返回IllegalStateException

Question

我正在尝试向数据库中添加元素，然后搜索特定条目。但是我得到了一个光标错误。这是我为DB类编写的代码。数据库只包含一列。提前谢谢。

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根本不起作用，你永远不会改变光标的位置，所以它总是真的会产生堆叠式

执行查询后，您应该询问光标是否为空，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，在你的情况下，因为你唯一的列是FirstName，所以你可以这样看：

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

请记住，如果更改数据库中的任何内容，则应升级数据库版本。