protected void buildcomponents() {
panelcard = new jpanel();
panelcard.setlayout(new cardlayout());
usermanpanel = new usermanpanel(parent, net);
panelcard.add(user_panel, usermanpanel);
// ...
add(panelcard);
}
// 要表示userpanel时:
cardlayout layout = (cardlayout)panelcard.getlayout();
layout.show(panelcard, user_panel);