Java Database Connectivity (JDBC)

I'm using netbeans to create my gui and want a combo box to hold the values returned from a select statement to my database.

However the panel holding the combo box is already drawn before the values can be added. This is because the username and password required to connect to the database are entered into the gui so a query cannot be made before the combo box is drawn.

When I try to use combobox.addItem(String) on making the connection and getting the results nothing is given in the combo box.

Any ideas?