您尚未在main方法的mainFrame的内容窗格中添加任何组件.添加面板的唯一时间是在此ActionListener中:

You have not added any components to the mainFrame's content pane in the main method. The only time a panel gets added is in this ActionListener:

    open.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {
            AssignToTransientPanel((JPanel) new NewAlbum());
            Container content = mainFrame.getContentPane();
            content.removeAll();
            content.add(transientPanel);
            content.validate();
            content.repaint();
        }
   });

仅当您单击打开"时才调用此函数，我想是偶然地将ActionListener添加到了打开的JMenuItem中，而不是newAlbum JMenuItem中.要在启动时添加内容，您需要在mainFrame.setVisible(true)行之前添加以下内容:

This is only getting called when "Open" is clicked as you have, I assume accidentally, added the ActionListener to the open JMenuItem rather than the newAlbum JMenuItem. To add content on startup you need to add something like this before the mainFrame.setVisible(true) line:

mainFrame.add(new NewAlbum());

顺便说一句，约定是Java源代码中的所有方法均以小写字母开头.对于您的方法，assignToTransientPanel会是一个更好的名称.

BTW, the convention is for all methods in Java source code to start with a lower case letter. assignToTransientPanel would be a better name for your method.