小程序，反复调试，未达愿望，望高手指点

Question

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class MyDialog extends JDialog
{
        public MyDialog(){
                super(new MyFrame(),"第一个Dialog窗体",true);
                Container container=getContentPane();
                container.add(new JLabel("这是一个对话框"));
                setSize(100,100);
    }
        public static void main(String args[]){
                new MyDialog();
    }
}
class MyFrame extends JFrame
{
        public MyFrame(){
                Container container=getContentPane();
                container.setLayout(null);
                JLabel jl=new JLabel("这是一个JFrame窗体");
                jl.setHorizontalAlignment(SwingConstants.CENTER);
                container.add(jl);
                setVisible(true);
                setBackground(Color.white);
                setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
                JButton bt=new JButton("弹出对话框");
                bt.setBounds(10,10,100,21);
                bt.addActionListener(new ActionListener(){
                        public void actionPerformed(ActionEvent e){
                                new MyDialog().setVisible(true);
                }
                });
                container.add(bt);
                setSize(500,500);
        }}
请问各位前辈为什么这个标签JLabel jl=new JLabel("这是一个JFrame窗体");为什么不能在Myframe中显示？
调试很久都不可以，望指点，不胜感激

Answer 1

import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 
public class MyDialog extends JDialog 
{ 
        public MyDialog(){ 
                super(new MyFrame(),"第一个Dialog窗体",true); 
                Container container=getContentPane(); 
                container.add(new JLabel("这是一个对话框")); 
                setSize(100,100); 
    } 
        public static void main(String args[]){ 
                new MyDialog(); 
    } 
} 
class MyFrame extends JFrame 
{ 
        public MyFrame(){ 
                Container container=getContentPane(); 
                container.setLayout(null); 
                JLabel jl=new JLabel("这是一个JFrame窗体"); 
                jl.setHorizontalAlignment(SwingConstants.CENTER); 
                jl.setBounds(60, 60, 150, 100);
                container.add(jl); 
                setVisible(true); 
                setBackground(Color.white); 
                setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE); 
                JButton bt=new JButton("弹出对话框"); 
                bt.setBounds(10,10,100,21); 
                bt.addActionListener(new ActionListener(){ 
                        public void actionPerformed(ActionEvent e){ 
                                new MyDialog().setVisible(true); 
                } 
                }); 
                container.add(bt); 
                setSize(500,500); 
        } } 

Answer 2

jl.setBounds(60, 60, 150, 100);
给你加 了一行，你看看 对不。

Answer 3

container.setLayout(null); 
把这个去了

Answer 4

控件没定位不能用null布局
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class MyDialog extends JDialog
{
        public MyDialog(JFrame owner){
                super(owner,"第一个Dialog窗体",true);
                Container container=getContentPane();
                container.add(new JLabel("这是一个对话框"));
                setSize(100,100);
    }
        public static void main(String args[]){
                new MyFrame();
    }
}
class MyFrame extends JFrame
{
private JFrame thisframe = this;
        public MyFrame(){
         setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
                Container container=getContentPane();
                container.setBackground(Color.white);
                container.setLayout(new FlowLayout()); //用null必须给每个控件定位，不然就重叠了
                JLabel jl=new JLabel("这是一个JFrame窗体");
                jl.setHorizontalAlignment(SwingConstants.CENTER);
                container.add(jl);
                JButton bt=new JButton("弹出对话框");
                bt.addActionListener(new ActionListener(){
                        public void actionPerformed(ActionEvent e){
                                new MyDialog(thisframe).setVisible(true);
                }
                });
                container.add(bt);
                setSize(500,500);
                setVisible(true);
        }
}