how to update panel with repaint()?

I have a panel-class:

public class MyPanel extends JPanel {
    Color myColor = Color.BLUE;
    String string = "Hello";
    
    public void update(String newString) {
        myColor = Color.GREEN;
        string = newString;
        repaint();
        
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setPaint(myColor);
        g2.drawString(string, 200, 100);        
    }
}

I want the panel to be repainted after I call the update()-method. But everything in update() works fine but not the repaint()-method which should repaint the panel. The panel is always the same with the old values. How can I update the values and show them in panel? In my frame-class I am calling the update() method. I checked if I get into that method and it works. So to mention: the calling of the update()-method cant be the problem. I also tried executing update() after some time, but repaint() never works.

public class MyFrame extends JFrame {
    
    public MyFrame() {
        setTitle("Task");
        setLayout(new BorderLayout());
        add(new MyPanel(), BorderLayout.CENTER);
    }
    
    public static void main(String [] args) {
        
        MyFrame frame = new MyFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(800, 600);
        frame.setVisible(true);
        
        MyPanel myPanel = new MyPanel();
        myPanel.update("new Hello");
        
        
    }
}

Your code is OK. Here's an example of it working with my suggestion. Don't use wildcard imports in real code like I did ;)

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

import java.io.*;

import javax.swing.*;

public class F extends JFrame {
    class MyPanel extends JPanel implements ActionListener {
        Color myColor = Color.BLUE;
        String string = "Hello";

        public MyPanel() {
            super();
            Timer t = new Timer(2000, this);
            t.start();
        }

        public void update(String newString) {
            // myColor = Color.GREEN;
            // Jazz it up
            // myColor = Color.GREEN;
            myColor = new Color((int) (Math.random() * Integer.MAX_VALUE));
            string = newString;
            repaint();

        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;
            g2.setPaint(myColor);
            g2.drawString(string, 200, 100);
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            String s = "" + (long) (Math.random() * Long.MAX_VALUE);
            update(s);
        }
    }

    private void setGui() {
        try {
            setLocation(0, 100);
            setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            Container cp = getContentPane();
            cp.add(new MyPanel());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    F f = new F();
                    f.setGui();
                    f.setSize(400, 200);
                    f.setVisible(true);
                }
            });
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}