NullPointerException Error with JPanel and CardLayout

问题

I am working on hotel management software for class and I am running into a few issues with my code. At this point, I am simply trying to add a JPanel I created in a separate class into my main gui. Any Help would be greatly appreciated. ~ Thanks!

Exception in thread "main" java.lang.NullPointerException
    at java.awt.Container.addImpl(Unknown Source)
    at java.awt.Container.add(Unknown Source)
    at hotelManage.HotelSystem.showGUI(HotelSystem.java:75)
    at hotelManage.HotelSystem.<init>(HotelSystem.java:27)
    at hotelManage.HotelSystem.main(HotelSystem.java:115)

Note: The error occurs at the line "jpanel.add("Room", room.getRoomPanel());"

The code: HotelSystem.java

import java.awt.*;
import java.awt.event.*;

import javax.swing.*;

public class HotelSystem extends JFrame implements ActionListener {
    /**
     * 
     */
    private static final long serialVersionUID = 1840835913045151061L;

    private JFrame mainFrame;
    private JPanel mainPanel;
    private JButton btnRoom;
    private JButton btnCustomer;
    private JButton btnOrder;
    private JButton btnSearch;
    RoomSystem room;

    //RoomSystem room = new RoomSystem();


    public HotelSystem(){
        prepareGUI();
        showGUI();
        registerListeners();


    }

    private void prepareGUI(){
        mainFrame = new JFrame("Hotel Management System");
        mainFrame.setSize(500,500);
        mainFrame.setLayout(new GridLayout(1,1));

        btnRoom = new JButton("Room Editor");
        btnCustomer = new JButton("Customer Editor");
        btnOrder = new JButton("Order");
        btnSearch = new JButton("Search");

        //main panel
        mainPanel = new JPanel();
        mainPanel.setLayout(new FlowLayout());

        mainFrame.add(mainPanel);
        mainFrame.setVisible(true);




    }

    private void showGUI(){
        final JPanel jpanel = new JPanel();
        jpanel.setBackground(Color.CYAN);
        jpanel.setSize(300,300);

        CardLayout cLayout = new CardLayout();
        cLayout.setHgap(5);
        cLayout.setVgap(5);
        jpanel.setLayout(cLayout);



        JPanel btnPanel = new JPanel(new FlowLayout());
        btnPanel.add(btnRoom);
        btnPanel.add(btnCustomer);
        btnPanel.add(btnOrder);
        btnPanel.add(btnSearch);
        jpanel.add("Button", btnPanel);
        jpanel.add("Room", room.getRoomPanel());

        mainPanel.add(jpanel);
        mainPanel.setVisible(true);  


    }

    public void registerListeners(){
        //register all buttons to self
        btnRoom.addActionListener(this);
        btnCustomer.addActionListener(this);
        btnOrder.addActionListener(this);
        btnSearch.addActionListener(this);

    } // end registerListeners

    public void actionPerformed(ActionEvent e){
        System.out.println(e.getActionCommand());
        //check all button presses and send
        //control to appropriate methods
       if (e.getSource() == btnRoom){

        } else if (e.getSource() == btnCustomer){

        } else if (e.getSource() == btnOrder){

        } else if (e.getSource() == btnSearch){

        } else {
            //lblOutput.setText("something went wrong");
        } // end if


    } // end actionPerformed


    public static void main(String[] args) {
        new HotelSystem();

    }


}

RoomSystem.java

import java.awt.*;

import javax.swing.*;



public class RoomSystem {


    //private JTextField roomName;
    private JButton btnEdit;
    private JPanel roomPanel;
    //private JButton roomCancel;

    //array here

    public RoomSystem(){
        btnEdit = new JButton("Create");

        JPanel roomPanel = new JPanel(new FlowLayout());
        roomPanel.add(btnEdit);


        roomPanel.setVisible(true);

    }



    public JPanel getRoomPanel() {
        return roomPanel;
    }


    public void setRoomPanel(JPanel roomPanel) {
        this.roomPanel = roomPanel;
    }




}

回答1:

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

You've never initialized room

RoomSystem room;

Even if you do initialize it RoomSystem room = new RoomSystem(), you still have another problem in your RoomSystem class. You have shadowed roomPanel, and therefore the class member is null, when you try and call getRoomPanel(). In your constructor, change

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

to 

roomPanel = new JPanel(new FlowLayout());

来源：https://stackoverflow.com/questions/23177175/nullpointerexception-error-with-jpanel-and-cardlayout