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);
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);
}