FlowLayout on top of GridLayout not working

Here are a few suggestions:

• Use a GridLayout for the top panel; in this case, zero means the number of rows is determined by the specified number of columns and the total number of components in the layout:

  JPanel north = new JPanel(new GridLayout(0, 9));
  

• Here’s an outline of how you can make your center panel have a reasonable initial size; note how you can draw relative to the current size:

  JPanel center = new JPanel() {
  
      private static final int N = 256;
      private static final String S = "Todo...";
  
      @Override
      protected void paintComponent(Graphics g) {
          super.paintComponent(g);
          int dx = (getWidth() - g.getFontMetrics().stringWidth(S)) / 2;
          int dy = getHeight() / 2;
          g.drawString(S, dx, dy);
      }
  
      @Override
      public Dimension getPreferredSize() {
          return new Dimension(N, N);
      }
  };
  

• You can construct your button names like this:

  for (int i = 0; i < 26; i++) {
      String letter = String.valueOf((char) (i + 'A'));
      buttons[i] = new JButton(letter);
      north.add(buttons[i]);
  }
  

• Make your panels instance variables and start on the event dispatch thread:

  EventQueue.invokeLater(new Runnable() {
  
      @Override
      public void run() {
          Hangman frame = new Hangman();
          frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
          frame.add(frame.north, BorderLayout.NORTH);
          frame.add(frame.center, BorderLayout.CENTER);
          frame.add(frame.south, BorderLayout.SOUTH);
          frame.pack();
          frame.setLocationByPlatform(true);
          frame.setVisible(true);
      }
  });