How to reload a panel without using CardLayouts?

Question

TLDR; :
I'm looking for a way to reload JPanels and its components.

I'd like to implement a dynamically changing JComboBox. This is the situation:
There is a button, which generates data in background ("generate content") here it is just some random numbers. After the data is generated, this data should be selectable inside a JComboBox.

/**
 * Initialize the contents of the frame.
 */
private void initialize() {
    frame = new JFrame();
    frame.setBounds(100, 100, 450, 300);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    panel = new JPanel();
    frame.getContentPane().add(panel, BorderLayout.CENTER);
    panel.setLayout(null);

    comboBox = new JComboBox<String>();

    comboBox.setBounds(39, 63, 144, 20);
    panel.add(comboBox);

    btnGenerateContent = new JButton("generate content");
    btnGenerateContent.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            comboContent = fillrandoms();
            comboBox = new JComboBox<>(fillrandoms());

        }
    });
    btnGenerateContent.setBounds(39, 11, 117, 23);
    panel.add(btnGenerateContent);
}

private static String[] fillrandoms() {
    String[] array = new String[10];
    for (int i = 0; i < 10; i++) {
        array[i] = "" + Math.random() * 100;

    }

    return array;
}

now when I execute the program, it will declare and initalize the combobox before there is data to display. My idea was now that when I hit the "generate content" button, this will also update the UI for the panel or the ComboBox. but when I insert panel.updateUI() or comboBox.updateUI() nothing happens.
After researches I found some ways to do it with CardLayouts, but I do not use them for my JComboBox, so I propably need another way. I don't even know if this is the common way or is there any other better way.

Answer 1

To update UI use method panel.revalidate(), panel.repaint(), or revalidate(). revalidate() and repaint() method are proper way to reflect UI update task.