Buttons in panels that doent work..

Question

hi all.
im currently developing an application where panels are switched on a frame.
each panel has its own actionlistener, and is working.
the problem is that the buttons i create in a panel get a null value when an action event occurs.
you can see the code below. (the problem is visible in the class hoofdmenu)
import java.awt.*;
import java.awt.event.*;
public class Metrio extends Frame implements WindowListener
     private Panel vorige;
     private hoofdMenu begin;
     private Leerling leer;
     private Vak vak;
     private Rapport rap;
     private Gebruiker gebruik;
     public static void main(String[] args)
          //g=f.getGraphics();
          //this.requestFocus();
          Metrio metrio =     new     Metrio();
          metrio.show();
     public Metrio()
          setTitle("cijfer registratie systeem");
          setSize(600,600);
          setLayout(new FlowLayout());
          addWindowListener(this);
          begin = new hoofdMenu(this);
          leer = new Leerling(this);
          vak = new Vak(this);
          rap = new Rapport(this);
          gebruik = new Gebruiker(this);
          setPaneel(0);
          //addActionListener(begin);
          requestFocus();
     public void     setPaneel(int paneel)
          if(vorige != null)
               remove(vorige);
               System.out.println(""+vorige+" begin="+begin);
          switch(paneel)
               case 0 : add(begin); vorige = begin; break;
               case 1 : add(leer); vorige = leer; break;
               case 2 : add(vak); vorige = vak; break;
               case 3 : add(rap); vorige = rap; break;
               case 4 : add(gebruik); vorige = gebruik; break;
          requestFocus();
          repaint();
     public void     paint(Graphics g)
     public void     windowOpened(WindowEvent e){}
     public void     windowClosing(WindowEvent e){System.exit(1);}
     public void     windowClosed(WindowEvent e){}
     public void     windowIconified(WindowEvent     e){}
     public void     windowDeiconified(WindowEvent e){}
     public void     windowActivated(WindowEvent     e){}
     public void     windowDeactivated(WindowEvent e){}
class hoofdMenu extends Panel implements ActionListener
     private     Metrio f;
     //private leerling leer;
     private     Button l, v, r,     g;
     public hoofdMenu(Metrio fr)
          f=fr;
          Button l = new Button("leerling");
          add(l);
          l.addActionListener(this);
          System.out.println("l2 = "+l);   // l is full here
          Button v = new Button("vak");
          add(v);
          v.addActionListener(this);
          Button r = new Button("rapport");
          add(r);
          r.addActionListener(this);
          Button g = new Button("gebruikers");
          add(g);
          g.addActionListener(this);
          System.out.println("l3 = "+l);
     public void     actionPerformed(ActionEvent     e)
          System.out.println("een     knop gedrukt!");
          System.out.println("e =     "+e.getSource());
          System.out.println("l =     "+l);             // l IS NULL HERE?????
          if(e.getSource() ==     l)
               System.out.println("ik ben er in!!!");
               f.setPaneel(1);
               System.out.println("leerling knop gedrukt!");
          System.out.println("v =     "+v);
          if(e.getSource() ==     v)
               f.setPaneel(2);
          if(e.getSource() ==     r)
               f.setPaneel(3);
          if(e.getSource() ==     g)
               f.setPaneel(4);
class Leerling extends Panel implements     ActionListener
     private     Metrio f;
     //private Panel ouder;
     private     Button b;
     public Leerling(Metrio fr)
          f=fr;
          //this.ouder = ouder;
          Button b = new Button("terug leerling");
          add(b);
          b.addActionListener(this);
     public void     actionPerformed(ActionEvent     e)
          f.setPaneel(0);
}thanks in advance,
robert

Answer 1

Your (l, v, r, g) buttons are re-declared locally in your hoofdMenu() constructor... that's why they are null-referenced outside of the method. So, your code should read: public hoofdMenu(Metrio fr)     {
   f=fr;
   l = new Button("leerling");
   add(l);
   l.addActionListener(this);
   System.out.println("l2 = "+l);   // l is full here
   v = new Button("vak");
   add(v);
   v.addActionListener(this);
   r = new Button("rapport");
   add(r);
   r.addActionListener(this);
   g = new Button("gebruikers");
   add(g);
   g.addActionListener(this);
   System.out.println("l3 = "+l);
}Hope this helped,
regards.