swing - Java: How to use a cardholder with multiple classes -

i'm programming in java, trying use cardholder in order switch between 2 jpanels each extension of own class. think understand basic concepts having errors in current revision, when calling classes. i'm getting null pointer exception , think it's structural problem i'm not sure how or why.

the main method points class

public class skeleton implements actionlistener{  jpanel cardholder; cardlayout cards; string carda = "a"; string cardb = "b";  jpanel jboard; jpanel jmenu;  jframe frame2;  board board; menu menu;  boolean menuset; boolean boardset;  timer timer;  public class switcher implements actionlistener{ string card;  switcher(string card){     this.card = card; }  @override public void actionperformed(actionevent e) {         cards.show(cardholder, card); } }  public skeleton(jframe frame){      jpanel menu = new menu();     jpanel board = new board();      jframe frame2 = frame;      timer = new timer(5, this);     timer.start();      cardholder = new jpanel();     cards = new cardlayout();     cardholder.setlayout(cards);     cardholder.add(menu, carda);     cardholder.add(board, cardb);     frame2.add(cardholder);     frame2.revalidate();     frame2.setvisible(true);  }  public jframe getskeleton(){     return frame2; }  public jpanel getcardholder(){     return cardholder; }  public void checkstatus(){     if (menuset == true){         new switcher(cardb);         boardset = false;     }     if (boardset == true){         new switcher(carda);         menuset = false;     } }  @override public void actionperformed(actionevent e) {     menuset = menu.getmenuset();     boardset = board.getboardset();     checkstatus();   }  } 

this board class, 1 of jpanels i'm trying switch between

public class board extends jpanel{  boolean boardset; menu menu = new menu();  public board(){ setbackground(color.white); }  public jpanel getpanel(){     return this; }  public void setboardset(boolean x){     boardset = x; }  public boolean getboardset(){     return boardset; } } 

here other jpanel class, contains button used switch other jpanel class. original starting jpanel used.

public class menu extends jpanel implements actionlistener{  boolean menuset;  public menu(){      setbackground(color.black);      jbutton button = new jbutton("hello");       button.addactionlistener(this);     this.add(button); }  public jpanel getpanel(){     return this; }  @override public void actionperformed(actionevent e) {     menuset = true; }  public void setmenuset(boolean x){     menuset = x; }  public boolean getmenuset(){     return menuset; } } 

like said, i'm getting null pointer exception. occuring on line of skeleton() class

menuset = menu.getmenuset(); 

the line above right after actionperformed event above (from timer), , have tested timer little, works doing basic print statements whenever try use 'menu' or 'board' instance inside actionperformed, null pointer exception.

i appreciate advice. idea way i'm doing may little convoluted. if has suggestions on better way helpful. main goal able call 2 separate classes 1 main class containing cardholder. way can separate code in order keep isolated , in order.

your skeleton class has "menu" member isn't set anywhere can see. constructor declares own "menu" local variable, local constructor , hides member. setting "menu" inside constructor won't set member. don't see anywhere else "menu" member set, unless i've missed or unless class in same package setting it.