JScrollPane未出现在JTextArea上

我正在尝试将JScrollPane添加到我的JTextArea中，但是不知何故，它不会出现. 我尝试根据JTextArea的尺寸调整其大小，但似乎不起作用.另外，请注意，我使用的是null布局，因为我希望在确定的位置上完全灵活地显示某些按钮和面板.

I'm trying to add the JScrollPane to my JTextArea, but somehow, it won't appear. I've tried resizing it according to the dimension of the JTextArea, but it doesn't seem to work. Also, notice that I'm using the null layout because I want the full-on flexibility of displaying certain buttons and panels at a pinpoint location.

import java.awt.Dimension;
import java.awt.BorderLayout;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;
import javax.swing.JScrollPane;
import javax.swing.ScrollPaneConstants;
import javax.swing.JTextArea;

public class PaneTester{

  private static JFrame frame;
  private static JPanel panel;
  private static JScrollPane scrollPane;
  private static JTextArea notificationBox;

  public static void main (String [] args){
    stage1(); 
    stage2();
  }

  private static void stage1(){
    createFrame();
    createPanel();
    frame.getContentPane().add(panel);
    panel.setVisible(true);
    frame.setVisible(true);
  }

  private static void stage2(){
    generateNotificationBox();
  }

  private static void createFrame(){
    frame = new JFrame();
    frame.setSize(new Dimension(900,700)); 
    frame.setExtendedState(JFrame.MAXIMIZED_BOTH); 
    frame.setLocationRelativeTo(null);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setResizable(true);
  }

  private static void createPanel(){
    panel = new JPanel();
    panel.setLayout(null);
    generateGridButtons();
  }

  private static void generateGridButtons(){
    short y = 0; 
    for(short i=0;i<4;i++){
      y += 60;
      short x = 500;
      for(short j=0;j<5;j++){
        JButton gridButton = new JButton();
        gridButton.setBounds(x, y,120,60);
        panel.add(gridButton);
        x += 140;
      }
    }    
  }
  public static void generateNotificationBox(){
    notificationBox = new JTextArea(10,10);
    notificationBox.setBounds(25, 25, 200, 400);
    scrollPane = new JScrollPane(notificationBox, ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS,
                                 ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS ); 

    Dimension d = new Dimension(notificationBox.getPreferredSize());
    scrollPane.getViewport().setPreferredSize(d);
    scrollPane.getViewport().add(notificationBox);
    panel.add(notificationBox);
    panel.repaint();
  }

}