class inputJPanel extends JPanel
          {
             public inputJPanel()
             {
               textArea = new JTextArea(25,70);
               scrollPane = new JScrollPane(textArea);
               scrollPane.setSize(800,600);
               add(scrollPane);
               repaint();
             }
             
             public JTextArea textArea;
             public JScrollPane scrollPane;  
          }

public void actionPerformed(ActionEvent evt)
          { 
              if(evt.getSource() instanceof JMenuItem)
              {
                 String arg = evt.getActionCommand();
              
                 if(arg.equals("新建"))
                 {
                    System.out.print(50);
                    
                    getContentPane().removeAll();
                                         
                    GridBagConstraints gbc = new GridBagConstraints();
                    gbc.fill = GridBagConstraints.HORIZONTAL;
                    gbc.anchor = GridBagConstraints.NORTH;
                    gbc.weightx = 100;
                    gbc.weighty = 0;
                    
                    p = new inputJPanel();
                    //getContentPane().setLayout();
                    getContentPane().add(p);                  
                 }
              }
          }

TEXTAREA在内部类里面，如果我不加在触发里面，他是显示的，但是一旦加在触发里面，他一定要触发以后，最大化窗口，他就会显示，再最小化他也在那里，同样在最大化窗口的状况下，触发，他也不显示，在最小化后他就显示了. 

删除或是添加控件后，需要重画一下
validate()或是updateUI()