问Java GUI:如何在JFrame上的JPanel中设置对JButton的关注？

EN

private void initialize() {

   // Launch the frame:
   frame = new JFrame();
   frame.setTitle("Welcome!");
   frame.setSize(520, 480);
   frame.setLocationRelativeTo(null);
   frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

   // Add the image:
   ImageIcon heroShotImage = new ImageIcon("heroShot.jpg");
   JPanel heroShotPanel = new JPanel();
   JLabel heroShot = new JLabel(heroShotImage);
   heroShotPanel.add(heroShot);

   // Create a panel to hold the "Start" button:
   JPanel submitPanel = new JPanel(new FlowLayout(FlowLayout.CENTER));

   // Create the "Start" button, which launches business logic and dialogs:
   JButton start = new JButton("Start");
   start.setToolTipText("Click to use library");
   start.setFocusable(true); // How do I get focus on button on App launch?
   start.requestFocus(true); // Tried a few things and can't get it to work.

   // Listen for user actions and do some basic validation:
   start.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
      // THE APP's LOGIC GOES HERE...
      }

   // Finish setting up the GUI and its components, listeners, and actions:
   submitPanel.add(start);

   frame.getContentPane().add(heroShotPanel, BorderLayout.NORTH);
       frame.getContentPane().add(submitPanel, BorderLayout.SOUTH);

}