looks like you may need a refresher course on how things work.
ArrayList list = new ArrayList(); Object[] arrayObject=list.toArray(); private JList receiverID_lst = new JList(arrayObject);
here, arrayObject is empty, ergo, receiverID_lst is also empty.
private DefaultListModel model = new DefaultListModel(); and, later model.addElement(rs.getString("User_ID")); all well and good, but means nothing('ether code'), model never being set.
ResultSet rs = stmt.executeQuery.... while(rs.next()){ ... list.add( rs ); ... } strange bit of code, that lot
receiverID_lst.setListData(arrayObject); well, arrayObject is still empty (there is no auto-recalculation)