problem in thread when calling a class member's method

"oleth" <>
28 Apr 2006 14:41:32 -0700

I have a problem when runing a thread. I have a gui application with a
menu and a menu item. The application extends JPanel and implements
ActionListener. It has several members and one specific member called
lets say "arrow1" which causes he problem. This member is an inner
class instance of a class called "ClassToCall". This class has a method
called "foo" which does nothing but write something to Sytem.out. This is called from inside run(). I get the following:

Exception in thread "Thread-2" java.lang.NullPointerException
        at MyProblem$
        at Source)

In order to replicate the problem try to build the code I have below
and when gui begins click Tranformations -> tranformationA and you get
the problem.

My major problem is I can't figure out why this is hapening and where
to start from

Any directions or ideas?

Thank you!

The code of the app is stripped to the bare minimun.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class MyProblem extends JPanel implements ActionListener
  private ClassToCall arrow1;

  /////// start GUI stuff
  private JFrame mainFrame;

  private JMenuBar menuBar;

  private JMenu fileMenu;
  private JMenuItem quitItem;
  private JMenuItem againItem;

  private JMenu transformationMenu;
  private JMenuItem tranformationAItem;

  private int horizontialDimension;
  private int verticalDimension;

  /////// end GUI stuff

  private int index;

  private boolean tranformationA;

  Graphics display;
  Font font;
  InnerThread in;
  Thread tr;

  public MyProblem()

  ///////////////// start set up the gui

    mainFrame = new JFrame("SwingTest");
    mainFrame.setSize(600, 400);

    // create a menu bar
    menuBar = new JMenuBar();

    /* create "File" menu fileMenu */
    fileMenu = new JMenu("File");
    quitItem = new JMenuItem("Quit");
    againItem = new JMenuItem("Do it Again");




    /* create "Tranformations" menu tranforma */
    transformationMenu = new JMenu("Transformations");
    tranformationAItem = new JMenuItem("Tranformation A");




    /* Put menuBar, Panel to the application's frame */
    mainFrame.getContentPane().add(menuBar, BorderLayout.NORTH);
    mainFrame.getContentPane().add(this, BorderLayout.CENTER);

    horizontialDimension = getSize().width;
    verticalDimension = getSize().height;


    display = getGraphics();
    font = new Font("TimesRoman", Font.BOLD, 36);

    tranformationA = false;
  ////////////////////ending set up the gui

    ClassToCall arrow1 = new ClassToCall();

  public void paint(Graphics g)
      g.drawLine(50, 50, 50, 100);

  public void update(Graphics g)
    g.fillRect(0, 0, horizontialDimension, verticalDimension);

  public void actionPerformed(ActionEvent ae)
    else if(ae.getSource()== tranformationAItem)

      tranformationA = true;
      in = new InnerThread();
      tr = new Thread(in);

  public static void main(String[] args)
    MyProblem mp = new MyProblem();

  public class InnerThread implements Runnable
    public void run()
      while (index < 50)
        //!!!!!!!!!!!!!!!!!!!! THE PROBLEM IS CAUSED HERE !!!!!!!!!!!!;
      index = 0;
      System.out.println("Thread finished");

  public class ClassToCall
    public ClassToCall()

    public void foo()


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