problem with changeVariables on ComboBox .

hi everybody , Please help me.
I have class SubComboBox extends ComboBox … and override changeVariables method

@Override
public void changeVariables(Object source, Map variables) {
String filterString = (String) variables.get(“filter”);
if (filterString != null && filterString.trim().length() > 0) {
if (oldText == null)
oldText = filterString;
else {
if (oldText.equals(filterString))
return;
else
oldText = filterString;
}
items.removeAllItems();

// i get list data from DB here …
for (Data data : lstdata) {
items.addItem(data );
}
}
super.changeVariables(source, variables);
}

when i’m typing , it will capture my text …and search data … And then i press arrow_up , dow to select item … comboBox.getValue() return null value … please help me , how i select this iteam ?

p/s : when i press arrow_up , down, shortcut not work, valuechange not work.