JPanel和CardLayout出现NullPointerException错误

jpanel.add("Room", room.getRoomPanel());

您从未初始化

room

RoomSystem room;

即使您确实对其进行了初始化

RoomSystem room = newRoomSystem()

RoomSystem

roomPanel

getRoomPanel()

// shadowing the class field roomPanelJPanel roomPanel = new JPanel(new FlowLayout());toroomPanel = new JPanel(new FlowLayout());