JPanel 未添加新组件

Question

I'm building a GUI where i need to add dynamically to a JPanel some Label, but the code is not working for some reason:

public class ChatClientGUI extends javax.swing.JFrame {

    /**
     * Creates new form ChatClientGUI
     * @param server : server remoto a cui connettersi
     */
    public ChatClientGUI(ChatServerIF server) {
        initComponents();
        ...    
        messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
        messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
        // those works, infact i see 2 "Mex:" label added to the JPanel
    }
    private void sendButtonActionPerformed(java.awt.event.ActionEvent evt) {                                           
        try {
            System.out.println("MEX SENT"); // I see this line in the terminal
            messagesPanel.add(new JLabel("New mex sent", SwingConstants.LEFT), BorderLayout.PAGE_START);
            // this does not work, nothing is added to the JPanel
            server.sendMessage(client, username.getSelectedItem(), messageText.getText());
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    } 

Answer 1

i need to add dynamically...

When you add components dynamically to a panel you need to invoke:

1. revalidate() and
2. repaint() (sometimes needed)

on the panel.

By default a component has a size of (0, 0) so there is nothing to paint.

The revalidate() will invoke the layout manager and the repaint() makes sure the entire panel is repainted

messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);
messagesPanel.add(new JLabel("Mex:", SwingConstants.LEFT), BorderLayout.PAGE_START);

Specifying the BorderLayout.PAGE_START doesn't seem to make sense. You can only add a single component to any area in the BorderLayout. Your panel must be using some other layout (not the BorderLayout) if you see multiple comonents. Therefore that constraint is not needed.