How come this...
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public class MainFrame extends JFrame {
    private JPanel panelOne;

    public MainFrame() {
        JLabel aLabel = new JLabel("A Label");

        panelOne = new JPanel(new FlowLayout(FlowLayout.CENTER));
        panelOne.add(aLabel);

        this.setLayout(new BorderLayout());
        this.add(panelOne, BorderLayout.NORTH);

        this.setSize(500, 500);
        this.setVisible(true);
    }
}
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Work just as well, what is the container for, it seems to work without
it. Any problems if I don’t use it. Thank you for you help.

public class MainFrame extends JFrame {
    private JPanel panelOne;

    public MainFrame() {
        JLabel aLabel = new JLabel("Label!!!");

        panelOne = new JPanel(new FlowLayout(FlowLayout.CENTER));
        panelOne.add(aLabel);

        Container container = this.getContentPane();
        container.setLayout(new BorderLayout());
        container.add(panelOne, BorderLayout.NORTH);

        this.setSize(500, 500);
        this.setVisible(true);
    }
}
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