Java problem

class ShowAFrame 
{
   
    public static void main(String args[])
    {
      new SimpleFrame();
    }
    
}

import java.awt.FlowLayout;
import javax.swing.JFrame;
import javax.swing.JButton;


class SimpleFrame extends JFrame 

{
    public SimpleFrame()
    {
        setTitle("Don't click the button!");
        setLayout(new FlowLayout());
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        add(new JButton("Panic"));
        setSize(300,100);
        setVisible(true);
    }
    
}