Different applications same code

Question

Hi!
I just made a tiny litte programm.
It's a JPanel, with JLabels an one JButton on it. The JLabels are instances of some other class.
The weird thing is, that starting the application, the same way!!! ... run as ..., the count of the Labels differ. Actually I added 4 JLabels and one button:
there are sometimes:
4 labels 1 button
1 label
2 labels
showing up.
WHY?

Answer 1

Well, it works, because, I initialize Kasten-objects
in the text-class.
Don't it?I don't know but when I run the following
package me.sabre150.research.scratch;
import java.util.*;
import javax.swing.*;
import java.awt.*;
public class test extends JPanel
    public Kasten [] ak;
    ArrayList<Kasten> a;// = new ArrayList<Kasten>();
    public test()
        super();
    public static void main(String[] args)
        JFrame f = new JFrame("test");
        f.setPreferredSize(new Dimension(300,400));
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        test myTest = new test();
        f.setContentPane(myTest);
        f.pack();
        f.setVisible(true);
        myTest.setLayout(new FlowLayout());
        Kasten k1 = new Kasten(9999, Color.yellow);
        myTest.addItem(k1);
        Kasten k2 = new Kasten(2);
        myTest.addItem(k2);
        JButton b = new JButton("button");
        myTest.addItem(b);
        Kasten kasten = new Kasten(123, Color.red);
        myTest.addItem(kasten);
        Kasten kasten1 = new Kasten(1, Color.green);
        myTest.addItem(kasten1);
        myTest.setVisible(true);
    public void addItem(JButton b)
        b.setText("in addItem");
        b.setOpaque(true);
        this.add(b);
    public boolean addItem(Kasten arg0)
        System.out.println(arg0.i); // add JLabel to
        arg0.setBorder(BorderFactory.createEtchedBorder());
        arg0.setOpaque(true);
        this.add(arg0);
        return a.add(arg0);
class Kasten extends JLabel
    int i;
    java.util.List<Kasten> a = new ArrayList<Kasten>();
    public Kasten(int in)
        this.setBackground(Color.blue);
        this.setForeground(Color.black);
        this.setBorder(BorderFactory.createBevelBorder(5));
        this.setBounds(20,100, 20,50);
        i = in;
        this.setText(""+i);
        setOpaque(true);
    public Kasten(int in, Color color)
        this.setBackground(color);
        this.setForeground(Color.black);
        this.setBorder(BorderFactory.createBevelBorder(5));
        this.setBounds(20,100, 50,50);
        i = in;
        this.setText(""+i);
        setOpaque(true);
}I get as output 9999
Exception in thread "main" java.lang.NullPointerException
        at me.sabre150.research.scratch.test.addItem(test.java:85)
        at me.sabre150.research.scratch.test.main(test.java:54)with the screen showing a frame with just one component BUT when I change the line    ArrayList<Kasten> a;// = new ArrayList<Kasten>();to    ArrayList<Kasten> a = new ArrayList<Kasten>();I get no exception and I see all the components.
So - who is right?