怎么样将一个类中的界面在一个容器中显示？？（可给一个JAVA仿例的源码吗，谢了）

//:LoginDialog.javaimport java.awt.event.*;import java.net.URL;import javax.swing.*;public class LoginDialog extends JDialog implements ActionListener{ private JButton ok,cancel; private JTextField user,pwd; private JLabel bg;//用Label对象来显示背景 public LoginDialog(){ this.setDefaultCloseOperation(JDialog.DISPOSE_ON_CLOSE); this.setSize(311,258); this.setLocationRelativeTo(null); this.getContentPane().setLayout(null); this.setResizable(false); //图片在网上,下载要一定的时间,所以等下才会更新背景 this.setTitle("等一下就看到背景了"); //初始化背景 bg = new JLabel(); bg.setHorizontalAlignment(JLabel.LEFT); bg.setVerticalAlignment(JLabel.TOP); bg.setLayout(null); // this.setContentPane(bg); user = new JTextField(); user.setBounds(100,105,188,22); this.getContentPane().add(user); pwd = new JPasswordField(); pwd.setLocation(user.getX(),user.getY()+30); pwd.setSize(user.getSize()); this.getContentPane().add(pwd); ok = new JButton("Submit"); ok.setLocation(118,pwd.getY()+38); ok.set