I am getting the error when compiling:

RoundButtonExample.java:38: error: non-static variable this cannot be referenced from a static context
        button = new RoundButton("Click me!");

The code is:

import javax.swing.*;
import java.awt.*;

public class RoundButtonExample {

    public class RoundButton extends JButton {
        public RoundButton(String label) {
            super(label);
            setOpaque(false);
            setContentAreaFilled(false);
            setFocusPainted(false);
            setBorderPainted(false);
        }
    
        @Override
        protected void paintComponent(Graphics g) {
            if ( getModel().isArmed()) {
                g.setColor(Color.lightGray);
            } else {
                g.setColor(getBackground());
            }
            g.fillOval(0, 0, getSize().width - 1, getSize().height - 1);
            super.paintComponent(g);
        }
    
        @Override
        protected void paintBorder(Graphics g) {
            g.setColor(getForeground());
            g.drawOval(0, 0, getSize().width - 1, getSize().height - 1);
        }
    }
    
    public static void main(String[] args) {
        JFrame frame = new JFrame("Round Button Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        RoundButton button;
        button = new RoundButton("Click me!");
        button.setPreferredSize(new Dimension(100, 100));
        
        frame.getContentPane().setLayout(new FlowLayout());
        frame.getContentPane().add(button);
        
        frame.setSize(200, 200);
        frame.setVisible(true);
    }
}

Why?