JTabbedPane gives null pointer exception

Question

I'm using JTabbedPane with different panels and i'm using gridLayout for the buttons array that i will add. I tried looping the buttons to much more easy code. But i got a null pointer exception i don't how did i get that kind of error. here's my code

import javax.swing.*;
import javax.swing.border.CompoundBorder;
import java.awt.*;
public class HotelRoomsGUI extends JPanel{
    private JTabbedPane mainJTP;
    private JPanel classicTab,deluxeTab,presidentialTab,classicSubPanel,deluxeSubPanel,presidentialSubPanel;
    private String classicRoomNo[] = {"101","102","103","104","105","106","107","108","109","101","111","112"};
    private String deluxeRoomNo[] = {"201","202","203","204","205","206","207","208","209","201","211","212"};
    private String presidentialRoomNo[] = {"301","302","303","304","305","306","307","308","309","301","311","312"};
    private JButton[] classicRoom, deluxeRoom, presidentialRoom;
    public HotelRoomsGUI(){

        setLayout(null);
        setBackground(new Color(90,90,90));
        add(tabbedPane());

    }
    public JPanel classic()
    {
        classicTab = new JPanel();
        classicTab.setBackground(new Color(70,70,70));
        classicTab.setLayout(null);
        classicSubPanel();
        return classicTab;
    }
    public JPanel classicSubPanel()
    {
        classicSubPanel = new JPanel();
        classicSubPanel.setBounds(10,10,605,455);
        classicSubPanel.setLayout(new GridLayout(4,3,10,10));
        classicSubPanel.setBackground(new Color(70,70,70));
        classicTab.add(classicSubPanel);
        rooms();
        return classicTab;
    }
    public JPanel deluxe()
    {
        deluxeTab = new JPanel();
        deluxeTab.setBackground(new Color(70,70,70));
        deluxeTab.setLayout(null);
        deluxeSubPanel();
        return deluxeTab;
    }
    public JPanel deluxeSubPanel()
    {
        deluxeSubPanel = new JPanel();
        deluxeSubPanel.setBounds(10,10,605,455);
        deluxeSubPanel.setLayout(new GridLayout(4,3,10,10));
        deluxeSubPanel.setBackground(new Color(70,70,70));
        deluxeTab.add(deluxeSubPanel);
        rooms();
        return deluxeSubPanel;
    }
    public JPanel presidential()
    {
        presidentialTab = new JPanel();
        presidentialTab.setBackground(new Color(70,70,70));
        presidentialTab.setLayout(null);
        presidentialSubPanel();
        return presidentialTab;
    }
    public JPanel presidentialSubPanel()
    {
        presidentialSubPanel = new JPanel();
        presidentialSubPanel.setBounds(10,10,605,455);
        presidentialSubPanel.setLayout(new GridLayout(4,3,10,10));
        presidentialSubPanel.setBackground(new Color(70,70,70));
        presidentialTab.add(presidentialSubPanel);
        rooms();
        return presidentialSubPanel;
    }
    public JTabbedPane tabbedPane()
    {
        UIManager.put("TabbedPane.selected", Color.ORANGE); 
        mainJTP = new JTabbedPane();
        mainJTP.setBackground(Color.WHITE); 
        mainJTP.setBounds(3,1,630,500);
        mainJTP.addTab("Classic",classic());
        mainJTP.addTab("Deluxe",deluxe());
        mainJTP.addTab("Presidential",presidential());
        return mainJTP;
    }
    public void rooms()
    {
        JButton presidentialRoom[] = new JButton[presidentialRoomNo.length];        
        JButton deluxeRoom[] = new JButton[deluxeRoomNo.length];
        JButton classicRoom[] = new JButton[classicRoomNo.length];
        for(int x = 0;x<classicRoomNo.length;x++){
            //classic rooms
            ImageIcon imageC = new ImageIcon("C:\\Users\\John\\workspace\\SystemTest\\src\\Images\\classicRooms.JPG"); // image
            classicRoom[x] = new JButton(classicRoomNo[x],imageC);
            classicRoom[x].setBackground(Color.WHITE);
            classicRoom[x].setBorder(new CompoundBorder(BorderFactory.createEtchedBorder(Color.WHITE,Color.GRAY),
                    BorderFactory.createEtchedBorder(Color.WHITE,Color.GRAY)));
            classicSubPanel.add(classicRoom[x]);
            //deluxe rooms
            ImageIcon imageD = new ImageIcon("C:\\Users\\John\\workspace\\SystemTest\\src\\Images\\deluxeRooms.JPG"); // image
            deluxeRoom[x] = new JButton(deluxeRoomNo[x],imageD);
            deluxeRoom[x].setBackground(Color.WHITE);
            deluxeRoom[x].setBorder(new CompoundBorder(BorderFactory.createEtchedBorder(Color.WHITE,Color.GRAY),
                    BorderFactory.createEtchedBorder(Color.WHITE,Color.GRAY)));
            deluxeSubPanel.add(deluxeRoom[x]);
            //presidential rooms
            ImageIcon imageP = new ImageIcon("C:\\Users\\John\\workspace\\SystemTest\\src\\Images\\presidentialRooms.JPG"); // image
            presidentialRoom[x] = new JButton(deluxeRoomNo[x],imageP);
            presidentialRoom[x].setBackground(Color.WHITE);
            presidentialRoom[x].setBorder(new CompoundBorder(BorderFactory.createEtchedBorder(Color.WHITE,Color.GRAY),
                    BorderFactory.createEtchedBorder(Color.WHITE,Color.GRAY)));
            presidentialSubPanel.add(presidentialRoom[x]);

        }
    }
}

and please give me some tips on how i can code more clearer and much better approach

Answer 1

You are getting a NullPointerException as deluxeSubPanel has not been instantiated when calling rooms() when calling:

deluxeSubPanel.add(deluxeRoom[x]);

As a general guideline, better to use a top-down approach when creating components with dependent components. This would make the creation order — JTabbedPane - JPanel -Sub-Components.

Answer 2

Take a look at my suggestion, this is your tabbedPane() method:

public JTabbedPane tabbedPane() {
    UIManager.put("TabbedPane.selected", Color.ORANGE);
    mainJTP = new JTabbedPane();
    mainJTP.setBackground(Color.WHITE);
    mainJTP.setBounds(3, 1, 630, 500);
    mainJTP.addTab("Classic", classic());
    mainJTP.addTab("Deluxe", deluxe());
    mainJTP.addTab("Presidential", presidential());
    rooms();  // I've inserted this, no panic!
    return mainJTP;
}

In the method above, the calls to classic(), deluxe() and presidential() already triggers invocations on rooms() method if you trace carefully. Instead, make one rooms() call as I've just done above. Remove calls to rooms() from elsewhere in your code.