[YouGina]

class okHandler implements ActionListener {
public void actionPerformed(ActionEvent e) {
RegistratiePanel reg = new RegistratiePanel();
reg.tijdStart.setText("Dit is een test");
}
}
Is this what you mean?

When I try this, nothing happens, the text "Dit is een test" is not sended to the JTextField, but there's also no error, although I would expect a stackoverflow?

The source with a couple of comments:

import java.awt.*;
import java.awt.event.*;

import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.JToolBar;
import java.awt.LayoutManager;

public class TijdsRegistratie extends JFrame {   
    public static void main(String[] args) {      
        JFrame frame = new TijdsRegistratie();
        frame.setSize(500, 500);
        frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
        frame.setTitle("Tijds Registratie");
        frame.setContentPane(new RegistratiePanel());
        frame.setVisible(true);
    }
}

class RegistratiePanel extends JPanel {
    private JToolBar bar;
    private JButton Ok, Cancel, Start, Stop;
    public JTextField tijdStart, tijdEind, Voor, Van;
    
    public RegistratiePanel() {
        
        setLayout(null);
        
        Ok = new JButton("Ok"); // This button is going to be used
        Cancel = new JButton("Cancel");
        Start = new JButton("Start");
        Stop = new JButton("Stop");
        
        Ok.addActionListener(new okHandler()); // The actionhandler is called
        
        bar = new JToolBar();
        
        bar.add(Ok);  // Button added to the JToolBar
        bar.add(Cancel);
        bar.add(Start);
        bar.add(Stop);
        
        add(bar);        
        bar.setFloatable(false);
        bar.setRollover(true);
        bar.setBounds(0, 0, 500, 30);
        
        tijdStart = new JTextField(10); // when Ok button is pushed, text should appear in here.
        tijdEind = new JTextField(10);
        Voor = new JTextField(10);
        Van = new JTextField(10);
        
        tijdStart.setBounds(100, 50, 300, 20);
        tijdEind.setBounds(100, 150, 300, 20);
        Voor.setBounds(100, 250, 300, 20);
        Van.setBounds(100, 350, 300, 20);
        
        add(tijdStart);
        add(tijdEind);
        add(Voor);
        add(Van);
                    
    }
}
class okHandler implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        RegistratiePanel reg = new RegistratiePanel(); // I guess this is instantiating the RegistratiePanel?
        reg.tijdStart.setText("Dit is een test"); // This is the text which should appear in the JTextField tijdStart
    }
}