java.lang.NullPointerException with JMenu and JMenuBar

Question

I have a JMenuBar that has one menu and three JRadioButtonMenuItems:

JMenuBar menuBar;
JMenu menu = new JMenu("Menu");
JRadioButtonMenuItem rbMenuItem;

I declare it:

    menu = new JMenu("A Menu");
   ButtonGroup group = new ButtonGroup();
rbMenuItem = new JRadioButtonMenuItem("A radio button menu item");
rbMenuItem.setSelected(true);

group.add(rbMenuItem);
menu.add(rbMenuItem);
menuBar.add(menu);

and I set it as the menu bar:

this.setJMenuBar(menuBar);

I run the file and it gives me:

Exception in thread "main" java.lang.NullPointerException
    at geometry.tools.main.RectangleFrame.<init>(RectangleFrame.java:47)
    at geometry.tools.main.RectangleFrame.main(RectangleFrame.java:95)
Java Result: 1

Line 47 is:

menuBar.add(menu);

and line 95 is:

 RectangleFrame thr = new RectangleFrame();

in the:

public static void main(String[] args){

    RectangleFrame thr = new RectangleFrame();
}

I don't really understand why. I haven't specified any value as null so I don't see why I get this error.

Thanks a lot.

Answer 1

menuBar is not initialized

JMenuBar menuBar = new JMenuBar();

Answer 2

You have to create an instance of your menubar.

JMenuBar menuBar = new JMenuBar()