JTabbedPane給出空指針異常

Question

我正在使用JTabbedPane與不同的面板，我正在使用gridLayout作為我將添加的按鈕數組。 我嘗試將按鈕循環到更簡單的代碼。 但我有一個空指針異常我不知道怎么會得到那種錯誤。 這是我的代碼

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]);

        }
    }
}

請給我一些提示，告訴我如何編寫更清晰，更好的方法

Answer 1

您正在獲取NullPointerException因為在調用時調用rooms()時尚未實例化deluxeSubPanel ：

deluxeSubPanel.add(deluxeRoom[x]);

作為一般准則，在創建具有相關組件的組件時，最好使用自上而下的方法。 這將創建創建順序 - JTabbedPane - JPanel -Sub-Components。

Answer 2

看看我的建議，這是你的tabbedPane()方法：

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;
}

在上面的方法中，如果仔細跟蹤classic(), deluxe()對classic(), deluxe()和presidential()調用已經觸發了對rooms()方法的調用。 相反，我正在上面做一個rooms()電話。 從代碼中的其他位置刪除對rooms()調用。