How do I set selectedIndex in this case

asked 2020-03-01 22:35:56 +0800

HotCat
1 ● 1

updated 2020-03-02 11:22:00 +0800

cor3000
6280 ● 2 ● 7

Here is my code

ArrayList<Cat> cats = new ArrayList<Cat>(); 
cats.add(new Cat("Mary", "1"));
cats.add(new Cat("Mark", "2"));
ListModelList(cats, true));
int i = cmbCat.getItemCount(); // i is 0
cmbCat.setSelectedIndex(0);

cmbCat is a Combobox. After I set the model of cmbCat, I want to set the SelectedIndex always fail. How can I achieve this? And why?