Moving initializations to a method generates nullpointer exceptions

Question

Hi,
I'm trying to tidy up some code in a class that extends JMenuBar, but I keep getting NullPointerExceptions. Honestly, I have no idea why the compiler doesn't recognized the objects I have just initialized.
See this:
(a couple more questions added in comments)
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.util.ArrayList;
import javax.swing.*;
public class MyMenuBar extends JMenuBar implements ActionListener
     private JMenuItem exitItem, newGameItem, stopGameItem;
     private JRadioButton shopping, selling, basic, keyboard;
     private JMenu editMenu, playingModeMenu;
     public MyMenuBar(MyFrame myFrame)
          // is there any way I can pass a mnemonic to a method in order to cut a few lines here too?     
          JMenu fileMenu = new JMenu("File");
          fileMenu.setMnemonic(KeyEvent.VK_F);
          this.add(fileMenu);
          editMenu = new JMenu("Edit");
          editMenu.setMnemonic(KeyEvent.VK_E);
          this.add(editMenu);        
          JMenu helpMenu = new JMenu("Help");
          helpMenu.setMnemonic(KeyEvent.VK_H);
          this.add(helpMenu);
          //First Menu
          // can I pass accelerators as well? How?
          this.addJMenuItem(newGameItem, "Start new game", true, fileMenu);
                //  below is where the exception is launched
          newGameItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_N, ActionEvent.CTRL_MASK));     
          this.addJMenuItem(stopGameItem, "Stop game", false, fileMenu);
          stopGameItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_W, ActionEvent.CTRL_MASK));          
          //Second Menu
          playingModeMenu = new JMenu("Playing mode");
          ButtonGroup playingModeGroup = new ButtonGroup();
          this.addRadioButton(shopping, "shopping", true, playingModeGroup, playingModeMenu);
          this.addRadioButton(selling,"selling", false, playingModeGroup, playingModeMenu);
          editMenu.add(playingModeMenu);
          JMenu interactionModeMenu = new JMenu("Interaction mode");
          ButtonGroup interactionModeGroup = new ButtonGroup();
          this.addRadioButton(basic,"Basic", true, interactionModeGroup, interactionModeMenu);
          this.addRadioButton(keyboard,"Keyboard", false, interactionModeGroup, interactionModeMenu);
          editMenu.add(interactionModeMenu);                         
     private void addRadioButton(JRadioButton radioButton, String text, boolean isSelected, ButtonGroup group, JMenu menu)
          radioButton = new JRadioButton(text);
          if (isSelected)
               radioButton.setSelected(true);
          group.add(radioButton);
          radioButton.addActionListener(this);
          menu.add(radioButton);
     private void addJMenuItem(JMenuItem item, String text, boolean isEnabled, JMenu menu)
          System.out.println(text);
          item = new JMenuItem(text);
          item.setEnabled(isEnabled);
          item.addActionListener(this);
          menu.add(item);
          System.out.println(text);
     public void actionPerformed(ActionEvent e1)
}Thanks!

Answer 1

These declarations have not been instantiated and so are all null:
    private JMenuItem exitItem, newGameItem, stopGameItem;
    private JRadioButton shopping, selling, basic, keyboard;When you send the reference to a method it shows up as null. Trying to reference
a variable whose value is null generates the exception. Either instantiate the
reference before sending it or try one of the two other approaches shown below.
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.util.ArrayList;
import javax.swing.*;
public class MyMenuBarRx extends JMenuBar implements ActionListener
    private JMenuItem exitItem, newGameItem, stopGameItem;
//    private JRadioButton shopping, selling, basic, keyboard;
    private JMenu editMenu, playingModeMenu;
    public MyMenuBarRx()
        // is there any way I can pass a mnemonic to a method
        // in order to cut a few lines here too?     
        JMenu fileMenu = new JMenu("File");
        fileMenu.setMnemonic(KeyEvent.VK_F);
        this.add(fileMenu);
        editMenu = new JMenu("Edit");
        editMenu.setMnemonic(KeyEvent.VK_E);
        this.add(editMenu);        
        JMenu helpMenu = new JMenu("Help");
        helpMenu.setMnemonic(KeyEvent.VK_H);
        this.add(helpMenu);
        //First Menu
        // can I pass accelerators as well? How?
        newGameItem = addJMenuItem("Start new game", true, fileMenu);
        //  below is where the exception is launched
        newGameItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_N,
                                        ActionEvent.CTRL_MASK));     
        stopGameItem = addJMenuItem("Stop game", false, fileMenu);
        stopGameItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_W,
                                        ActionEvent.CTRL_MASK));          
        //Second Menu
        playingModeMenu = new JMenu("Playing mode");
        ButtonGroup playingModeGroup = new ButtonGroup();
        this.addRadioButton("shopping", true, playingModeGroup,
                             playingModeMenu);
        this.addRadioButton("selling", false, playingModeGroup,
                            playingModeMenu);
        editMenu.add(playingModeMenu);
        JMenu interactionModeMenu = new JMenu("Interaction mode");
        ButtonGroup interactionModeGroup = new ButtonGroup();
        this.addRadioButton("Basic", true, interactionModeGroup,
                             interactionModeMenu);
        this.addRadioButton("Keyboard", false, interactionModeGroup,
                             interactionModeMenu);
        editMenu.add(interactionModeMenu);                         
    private void addRadioButton(String text, boolean isSelected,
                                ButtonGroup group, JMenu menu)
        JRadioButton radioButton = new JRadioButton(text);
        if (isSelected)
            radioButton.setSelected(true);
        group.add(radioButton);
        radioButton.addActionListener(this);
        menu.add(radioButton);
    private JMenuItem addJMenuItem(String text, boolean isEnabled, JMenu menu)
        System.out.println(text);
        JMenuItem item = new JMenuItem(text);
        item.setEnabled(isEnabled);
        item.addActionListener(this);
        menu.add(item);
        System.out.println(text);
        return item;
    public void actionPerformed(ActionEvent e1)
    public static void main(String[] args)
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setJMenuBar(new MyMenuBarRx());
        f.setSize(200,200);
        f.setLocation(200,200);
        f.setVisible(true);
}