setContentPane() works but getContentPane().add() doesn't work

Question

This is my code. When I setContentPane(panel) it works, but when I getContentPane().add(panel) it doesn't work.

Why doesn't the second method work? (It doesn't show me anything)

public MyFrame() {

    final JFrame frame = new JFrame();
    frame.setLayout(new BorderLayout());
    frame.setResizable(false);
    frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    frame.setFont(new Font("Comic Sans MS", Font.BOLD, 12));

    JMenuBar mnuBar = new JMenuBar();

    JMenu mnu1 = new JMenu("aaa");
    mnu1.setMnemonic(KeyEvent.VK_E);

    JMenuItem menuItem = new JMenuItem("Insert Places", KeyEvent.VK_T);
    menuItem.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent arg0) {

            count = 0;
            jt = new ArrayList[4];
            for (int i = 0; i < 4; i++)
                jt[i] = new ArrayList<JTextField>();

            panel = new JPanel();
            panel.setLocation(0, 0);
            panel.setSize(d.width, d.height);
            panel.setLayout(null);


            JButton add = new JButton("lllll");
            add.setSize(120, 80);
            add.setLocation(250, 100);

            add.addActionListener(new ActionListener() {

                @Override
                public void actionPerformed(ActionEvent arg0) {

                    for (int i = 0; i < 4; i++) {
                        JTextField jt1 = new JTextField(20);
                        jt1.setSize(150, 30);
                        jt1.setLocation(450 + i * 200, 80 + count * 50);
                        jt1.setVisible(true);
                        jt[i].add(jt1);
                        panel.add(jt1);
                    }
                    count++;

                    frame.repaint();

                }
            });
            panel.add(add);

            JButton confirm = new JButton("ccc");
            confirm.setSize(120, 80);
            confirm.setLocation(250, 200);

            confirm.addActionListener(new ActionListener() {

                @Override
                public void actionPerformed(ActionEvent arg0) {

                    count = 0;
                    for (int j = 0; j < jt[0].size(); j++) {
                        try {

                            connect.insertPlace(Integer.parseInt(jt[0]
                                .get(j).getText()), jt[1].get(j).getText(),
                                Integer.parseInt(jt[2].get(j).getText()),
                                Integer.parseInt(jt[3].get(j).getText()));
                            count++;
                        } catch (Exception e) {
                            JOptionPane.showMessageDialog(null, e, "ERROR",
                                0, null);
                        }
                    }

                    JLabel countL = new JLabel("" + count
                        + "    ??? ??? ??     ");
                    countL.setSize(100, 100);
                    countL.setLocation(250, 350 + jt.length * 10);
                    panel.add(countL);
                    frame.repaint();

                }
            });
            panel.add(confirm);
            frame.getContentPane().add(panel);

        }
    });
    mnu1.add(menuItem);
    mnuBar.add(mnu1);
    frame.setJMenuBar(mnuBar);
    frame.pack();
    frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
    frame.setVisible(true);
}

Answer 1

The Layout of the existing contentpane is such, that your panel isn't sized correctly and doesn't display. Compare the layout of the existing contentpane, the JFrame and your JPanel and notice the differences.

Answer 2

Taken from the tutorials:

Specify the component's location (for example, BorderLayout.LINE_END) as one of the arguments to the add method. If this component is missing from a container controlled by a BorderLayout object, make sure that the component's location was specified and no another component was placed in the same location.

From what I can see, you have

frame.setLayout(new BorderLayout());

But then, you don't specify its location.

frame.getContentPane().add(panel);

Try, instead,

frame.getContentPane().add(panel, BorderLayout.CENTER);

---

Also, I'm not sure if the order of the following operations is relevant

panel = new JPanel();
panel.setLocation(0, 0);
panel.setSize(d.width, d.height);
panel.setLayout(null);

But you may set the layout directly on the constructor.

panel = new JPanel(null);

And, since your panel is going to be inserted in a container controlled by a layout manager, you shouldn't need to specify its location. As to its dimensions, you may specify its preferred size instead, although this isn't deemed as a good practice by some.

panel.setPreferredSize(new Dimension(d.width, d.height));