为什么ComboBox显示不出来,该怎么解决

/* * *  */import java.awt.BorderLayout;import java.awt.Dimension;import java.awt.Graphics;import java.awt.Graphics2D;import java.awt.Toolkit;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.awt.event.MouseAdapter;import java.awt.event.MouseEvent;import java.awt.*;import javax.swing.*;public class Splash_Window extends   JWindow {        ImageIcon imageIcon;    public   Splash_Window(final JFrame f,int waitTime)     {                 super(f);         imageIcon=new   ImageIcon(getClass().getResource("/").getPath().replace("%20", " ")+"image/xinlangweibo.jpg");        f.setSize(this.imageIcon.getIconWidth(), this.imageIcon.getIconHeight());        f.setTitle("请选择");                f.setDefaultCloseOperation(3);        f.setUndecorated(true);                if(imageIcon==null)            System.out.println("imageIcon null ");        JPanel p=new JPanel(){            public void paintComponent(Graphics g){//重写paintComponent                super.paintComponent(g);//继承的绘制组件方法                                Graphics2D gg=(Graphics2D)g;                if(imageIcon !=null){//绘制背景                    gg.drawImage(imageIcon.getImage(),0,0,f.getWidth(),f.getHeight(),f);                    System.out.println("draw ");                }                            }        };        p.setLayout(null);//空布局  任意摆放        String [] selectItem=new String[]{"请选择关联文件","关联数据库","关联已存文件","默认"};        JComboBox   ComboBox_Select=new JComboBox(selectItem);        ComboBox_Select.setVisible(true);        p.add(ComboBox_Select);        ComboBox_Select.setLocation(f.getWidth()/2, f.getHeight()/2);        Dimension   screenSize   = Toolkit.getDefaultToolkit().getScreenSize();         f.setBounds((screenSize.width-this.imageIcon.getIconWidth())/2, (screenSize.height-this.imageIcon.getIconHeight())/2, this.imageIcon.getIconWidth(), this.imageIcon.getIconHeight());        System.out.println((screenSize.width-this.imageIcon.getIconWidth())/2+" "+(screenSize.height-this.imageIcon.getIconHeight())/2);        System.out.println(this.imageIcon.getIconWidth()+" "+this.imageIcon.getIconHeight());        f.setContentPane(p);//不可直接用f.add(p);    //        f.add(ComboBox_Select);        f.setVisible(true);        pack();         f.addMouseListener(new   MouseAdapter()         {             public   void   mousePressed(MouseEvent   e) {                 setVisible(false);                 System.exit(0);                dispose();             }         });             }    public static void main(String [] args){        final JFrame f=new JFrame();        Splash_Window sWindow=new Splash_Window(f,10000);    }        }