Hi,

normally, I would prefer to call pack(), but if there is a reason to set
the bounds to fixed values, try this:

    validate();        // let the layout manager do his job
   

Dieter

Here's the solution:

I have to write:

setBounds(0,0,800,600);
setVisible(true);

instead of first setVisible and then setBounds....
I really don't get it

It works for me after I swapped the following
lines like this :

setBounds(0,0,800,600);
setVisible(true);

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

public class Hoofdscherm extends JFrame
{
// private Beheerder deBheerder;
private JButton btnMatrixVolledig, btnMatrixVariabelen, btnMatrixVgl;

// public Hoofdscherm(Beheerder deBeheerder)
public Hoofdscherm()
{
 super("SimplexMethode");
 // this.deBheerder = deBeheerder;
 ActionEventHandler handler = new ActionEventHandler();

 Container c = getContentPane();
 c.setLayout(new FlowLayout());

 btnMatrixVolledig = new JButton("Volledige matrix ingeven");
 btnMatrixVolledig.addActionListener(handler);
 c.add(btnMatrixVolledig);

 pack();
 setVisible(true);
 setBounds(0,0,800,600);
}

private class ActionEventHandler implements ActionListener
   {
    public void actionPerformed(ActionEvent e)
    {

     }
  }

  public static void main(String[] args)
  {
   Hoofdscherm h = new Hoofdscherm();
  }
}

1) You need to improve your 'small self contained,
compileable example' style, I had to comment
out various lines and add a main.

2) Learn some Layout managers, you may be
usedf to AWT where default llayout was FlowLayout,
but in Swing it is BorderLayout, does not go down
well with adding multiple components the way you
added the button.

HTH

--
Andrew Thompson
* http://www.PhySci.org/ PhySci software suite
* http://www.1point1C.org/ 1.1C - Superluminal!
* http://www.AThompson.info/andrew/ personal site

public class Hoofdscherm extends JFrame

...

There are two problems with that.

1) You should not set visible from within a constructor.

    Hoofdscherm tmp = new Hoofdscherm( thing );
    tmp.pack();
    tmp.show();

2) You should not be explicitly setting bounds on a JFrame. Instead call
pack() on it. Leave the numbers up to layout managers.