Java Swing。菜单/选项框/的ActionListeners

Question

使用MenuBar编译并弹出panelMain。我可以推送编辑并转到subMenu，然后单击“首选项”。它出现了一个错误，指出“线程中的异常”AWT-EventQueue-0“java.lang.NullPointerException”。单击“首选项”后我想要发生的是创建一个新的JFrame，它为我提供了为mainPanel选择三种不同背景颜色的选项。

我添加了一张关于我要创建

的图表的绘图

P4App类

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class P4App extends JFrame
{
    P4File fileFrame;
    JPanel panelMain;
    JPanel panelselect;
    public P4App()
    {
        panelMain = new JPanel();
        panelMain.setLayout(new BorderLayout());

        JMenuBar menubar = new JMenuBar();
        JMenu edit = new JMenu("Edit");
        edit.setMnemonic(KeyEvent.VK_E);
        menubar.add(edit);

        JMenuItem pref = new JMenuItem("Preferences");
        pref.setMnemonic(KeyEvent.VK_P);
        edit.add(pref);

        setJMenuBar(menubar);

        this.setSize(400,150);
        this.setLocation(400,200);
        panelMain.setBackground(Color.RED);
        this.setVisible(true);
        this.setDefaultCloseOperation(EXIT_ON_CLOSE);

        fileFrame = new P4File(this);
        pref.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    remove(menubar);
                    remove(pref);
                    add(panelselect);
                    revalidate();
                    repaint();
                }
            });
    }

    public static void main(String [] args) {
        new P4App();
    }
}

P4File类

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class P4File extends JFrame

{
    JRadioButton radiored;
    JRadioButton radioblue;
    JRadioButton radiogreen;
    JPanel panelMain;
    JButton buttonok;
    JButton buttoncancel;
    JPanel panelselect;
    JPanel pbuttons;
    P4App parent;
    public P4File(P4App parent)
    {
        super("P4File");
        this.parent = parent;

        ActionHandler actionHandler = new ActionHandler();

        panelselect = new JPanel();
        panelselect.setLayout(new GridLayout(4,1,5,5));

        panelselect.add(radiored = new JRadioButton("Red"));
        panelselect.add(radioblue = new JRadioButton("Blue"));
        panelselect.add(radiogreen = new JRadioButton("Green"));

        ButtonGroup group = new ButtonGroup();
        group.add(radiored);
        group.add(radioblue);
        group.add(radiogreen);

        panelselect.add(radiored);
        panelselect.add(radioblue);
        panelselect.add(radiogreen);

        pbuttons = new JPanel();
        pbuttons.setLayout(new FlowLayout(FlowLayout.RIGHT,2,2));
        pbuttons.add(buttonok = new JButton("OK"));
        pbuttons.add(buttoncancel = new JButton("CANCEL"));

        panelselect.add(buttonok, BorderLayout.SOUTH);
        panelselect.add(buttoncancel, BorderLayout.SOUTH);

        buttoncancel.addActionListener(new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {

                    panelMain.setBackground(Color.RED);
                    System.exit(0);
                }
            });
    }

    private class ActionHandler implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            setColor();
        }
    }

    public void setColor() {
        if(radiored.isSelected())
        {
            setBackground(Color.RED);
        }

        if(radiogreen.isSelected())
        {
            setBackground(Color.GREEN);
        }

        if(radioblue.isSelected())
        {
            setBackground(Color.BLUE);
        }
    }
}