java.lang.NullPointerException with JMenu and JMenuBar -

i have jmenubar has 1 menu , 3 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 set menu bar:

this.setjmenubar(menubar); 

i run file , gives me:

exception in thread "main" java.lang.nullpointerexception     @ geometry.tools.main.rectangleframe.<init>(rectangleframe.java:47)     @ 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 understand why. haven't specified value null don't see why error.

thanks lot.

menubar not initialized

jmenubar menubar = new jmenubar();