import javax.swing.*;
import java.awt.*;
public class MY extends JFrame{
    JLabel jl1;   
    public MY(){
        super("A");
        
        jl1=new JLabel("欢迎");
        jl1.setBounds(60,60,60,60);
        Container ct=this.getContentPane();        
        ct.setLayout(null);
        ct.add(jl1);
        ct.setSize(300,150);
        ct.setVisible(true);
    }

    public static void main(String args[]){
        MY my=new MY();   
    }
   
}

import javax.swing.*;
import java.awt.*;
public class MY extends JFrame{
    JLabel jl1;     
    public MY(){
                 
        jl1=new JLabel("欢迎");
        jl1.setBounds(60,60,60,60);
        Container ct=this.getContentPane();        
        ct.add(jl1);
        this.setSize(300,150);
        this.setVisible(true);
    }
 
    public static void main(String args[]){
        MY my=new MY();     
    }
     
}

楼主 这个程序也可以这样写
import javax.swing.*;
import java.awt.*;
public class MY extends JFrame{
    JLabel jl1;   
    public MY(){
        super("A");
        
        jl1=new JLabel("欢迎");
        jl1.setBounds(60,60,60,60);
         
        setLayout(null);
        add(jl1);
        setSize(300,150);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); //注意 这句一定要 说明用户点击关闭窗口按钮时要执行什么
    }

    public static void main(String args[]){
        MY my=new MY();   
    }
   
}