为什么窗口的背景颜色不能变化

import java.awt.Button;
import java.awt.Color;
import java.awt.Font;
import java.awt.Label;
import java.awt.TextField;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

import javax.swing.JFrame;
import javax.swing.JOptionPane;




public class My {
    public static void main(String[] args) {
        new A();

    }

}
class A{
    JFrame j=new JFrame("登陆验证");
    Button ok=new Button("验证");
    Button no=new Button("重置");
    Label be=new Label("密码");
    Label la=new Label("用户名");
    TextField  id=new TextField(15);
    TextField  pass=new TextField(15);
    public A(){
        Font f=new Font("隶书",Font.BOLD,20);
                //明明设置了背景颜色，可是执行后窗口背景颜色就是没变化
        j.setBackground(Color.CYAN);

        j.setBounds(300,300,400,300);
        j.setLayout(null);
        j.setVisible(true);
        j.addWindowListener(new Window());
        ok.setBounds(80,200,50,30);
        no.setBounds(200,200,50,30);
        ok.setFont(f);
        no.setFont(f);
        j.add(ok);
        j.add(no);
        no.addActionListener(new NoActionListener());
        ok.addActionListener(new OkActionListener());
        la.setBounds(70,60,80,50);
        be.setBounds(70,100,80,50);
        la.setFont(f);
        be.setFont(f);
        j.add(la);
        j.add(be);
        pass.setEchoChar('*');
        id.setBounds(160,70,120,30);
        pass.setBounds(160,110,120,30);
        //添加文本框
        j.add(id);
        j.add(pass);
        
    }
    class NoActionListener implements ActionListener
    {

        public void actionPerformed(ActionEvent arg0) {
            id.setText("");
            pass.setText("");
        }
        
    }
    class OkActionListener implements ActionListener
    {

        public void actionPerformed(ActionEvent e) {
            // TODO Auto-generated method stub
            if(id.getText().equals("123456") && pass.getText().equals("chengkailei"))
            {
                JOptionPane.showMessageDialog(null,"登陆成功");

            }
            else
            {
                JOptionPane.showMessageDialog(null,"登陆失败");
            }
        }
         
    }
}
class Window extends WindowAdapter
{

    @Override
    public void windowClosing(WindowEvent arg0) {
        System.exit (0);
    }
   
}