Android android.database.CursorIndexOutOfBoundsException:Index -1 requested, with a size of 1

Android中数据库处理使用cursor时，游标不是放在为0的下标，而是放在为-1的下标处开始的。

也就是说返回给cursor查询结果时，不能够马上从cursor中提取值。

下面的代码会返回错误

User u = null;  
SQLiteDatabase db = dbHelper.getReadableDatabase();  
Cursor cursor = db.rawQuery("select * from user where id = ?", new String[] { id.toString() });  
Integer uid = cursor.getInt(cursor.getColumnIndex("id"));  
String uname = cursor.getString(cursor.getColumnIndex("name"));  
float uamount = cursor.getFloat(cursor.getColumnIndex("amount"));  
u = new User(uid, uname, uamount);cursor.close();  

正确的用法

User u = null;  
SQLiteDatabase db = dbHelper.getReadableDatabase();  
Cursor cursor = db.rawQuery("select * from user where id = ?", new String[] { id.toString() });  
if (cursor.moveToFirst())  
{  
 Integer uid = cursor.getInt(cursor.getColumnIndex("id"));  
 String uname = cursor.getString(cursor.getColumnIndex("name"));  
 float uamount = cursor.getFloat(cursor.getColumnIndex("amount"));  
 u = new User(uid, uname, uamount);  
}  
cursor.close();