Blank Frame In Java

import javax.swing.*;
public class swingstart
{
    public static void main(String[] args)
    {
        SimpleFrame frame=new SimpleFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}
class SimpleFrame extends JFrame
{
    public SimpleFrame()
    {
        setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
    }
    static final int DEFAULT_WIDTH=400;
    static final int DEFAULT_HEIGHT=300;
}