Something

import javax.swing.*;
import java.awt.*;
/**
*
* @author Martijn
*/
public class Rekenmachine extends JFrame{
    private JTextField textvak;
    private JPanel knoppenpanel;
 
    public Rekenmachine(){
 
        knoppenpanel = new Knoppen(this);
        textvak = new JTextField(15);
        add(textvak);
        add(knoppenpanel);
        textvak.setText("123"); // this works 
        textvak.setEnabled(false);
        setLayout(new FlowLayout());
    }
 
    public void setTextVak(){
        String tekst = textvak.getText();
        textvak.setText("blablabllalbalba"); // keeps showing 123
        System.out.println(textvak.getText()); // shows blablabllalbalba
        
    }
}

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import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
/**
*
* @author Martijn
*/
public class Knoppen extends JPanel {
    private JButton[] buttons = new JButton[16];
    
    
    public Knoppen(Rekenmachine r){
        KnopHandler kh = new KnopHandler(r);
        
        for(int i = 0; i < buttons.length; i++){
            buttons[i] = new JButton();
            buttons[i].setText("" + (i));
            if(i == 10){
                buttons[i].setText("C");
            }
            if(i == 11){
                buttons[i].setText("/");
            }
            if(i == 12){
                buttons[i].setText("+");
            }
            if(i == 13){
                buttons[i].setText("-");
            }
            if(i == 14){
                buttons[i].setText("*");
            }
            if(i == 15){
                buttons[i].setText("=");
            }
           add(buttons[i]);
           buttons[i].addActionListener(kh);
        }  
        setLayout( new GridLayout(4,5)); 
    }
    public class KnopHandler implements ActionListener{
    	private Rekenmachine r;
    	
    	public KnopHandler(Rekenmachine r)
    	{
    		this.r = r;
    	}
        
        public void actionPerformed(ActionEvent e){
            if(e.getSource() == buttons[1]){
                r.setTextVak();   
            }
            if(e.getSource() == buttons[2]){
                System.out.println("bla");
            }
 		}
    }
}



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import javax.swing.*;
import java.awt.*;
/**
*
* @author Martijn
*/
public class Main {
 
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        Rekenmachine r = new Rekenmachine();
        r.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        r.setSize(200, 200);
        r.setTitle("Rekenmachine");
        r.setVisible(true);
    }
}