public class Agenda{
    private  JFrame frame;
    private  String[] agenda;

    public Agenda(){
        iniciaArray();

        JPanel pane = retornaPanel(agenda);
        pane.setVisible(false);

        frame = new JFrame();
        frame.setSize(300,300);
        frame.setContentPane(new JPanel(new BorderLayout()));
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JButton btn = new JButton("Mostrar");
        frame.getContentPane().add(btn, BorderLayout.NORTH);
        frame.getContentPane().add(pane, BorderLayout.CENTER);

        btn.addActionListener(new Listener(pane));

        frame.setVisible(true);
    }

    public JPanel retornaPanel(String[] agenda){
        JPanel pane = new JPanel();
        DefaultListModel listModel = new DefaultListModel();
        JList list = new JList(listModel);

        for(int i = 0; i < agenda.length; i++)
            listModel.addElement(agenda[i]);
        JScrollPane scroll = new JScrollPane(list);
        pane.add(scroll);

        return pane;

    }

    public void iniciaArray(){
        agenda = new String[10];

        for(int i = 0; i < 10; i++)
            agenda[i] = "Elemento" + i;
    }

}

public class Listener implements ActionListener {
    private JPanel pane;
    public Listener(JPanel p){
        pane = p;
    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        pane.setVisible(true);
    }

}