Multiple Lines with a FlowLayout within a SOUTH region in a BorderLayout

Hi

My Problem is, that I want to have a my Buttons in the SOUTH in a
Border Layout.
There, the buttons are aligned in a FlowLayout. When the user
changes the size of the Window, the buttons are repainted.
If there is not enough space, the Buttons disapear, instead of what
I want to have:
The SOUTH region enlarges and two rows of buttons appear.

Any Idea how to perform the above task?

Here the source I use to test the behaviour:

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

public class MultiLineSouth {
  public static void main(String[] args) {
    new MultiLineSouth().top();
  }

  void top() {
    JFrame jf = new JFrame("Multiple Lines in the South");
    JPanel mainPanel = makeMainPanel();
    jf.add(mainPanel);
    jf.setSize(300, 300);
    jf.setVisible(true);
  }

  JPanel makeMainPanel() {
      JPanel mainPanel = new JPanel();
      mainPanel.setLayout(new BorderLayout());
      populateMainPanel(mainPanel);
      return mainPanel;
  }

  void populateMainPanel(JPanel mainPanel) {
     JLabel centerLabel = new JLabel("center");
     mainPanel.add(centerLabel, BorderLayout.CENTER);
     JPanel southPanel = makeSouthPanel();
     mainPanel.add(southPanel, BorderLayout.SOUTH);
  }

  JPanel makeSouthPanel() {
    JPanel southPanel = new JPanel();
    //southPanel.setLayout(null); // which to use here?
    populateSouthPanel(southPanel);
    return southPanel;
  }

  void populateSouthPanel(JPanel southPanel) {
    JPanel mehrZeilenPanel = makeMultiLinePanel();
    southPanel.add(mehrZeilenPanel);
  }

  JPanel makeMultiLinePanel() {
    JPanel mzp = new JPanel();
    mzp.setLayout(new FlowLayout());
    populateMultiLinePanel(mzp);
    return mzp;
  }

  void populateMultiLinePanel(JPanel mzp) {
    mzp.add(new JButton("Hello"));
    mzp.add(new JButton("World"));
    mzp.add(new JButton("OK"));
    mzp.add(new JButton("Cancel"));
  }

} // end of class MultiLineSouth

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thx