An implementation of circular progress bar concept in Java/Swing using the 2D Graphics, Shapes API

CircleProgressTest.java

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.geom.AffineTransform;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class CircleProgressTest {

    public static void main(String[] args) {
        new CircleProgressTest();
    }

    public CircleProgressTest() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private double progress = 0d;
        private double radius = 90;
        private CircleProgress cp;

        public TestPane() {
            cp = new CircleProgress(radius, 20, 0d);
            Timer timer = new Timer(50, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    if (progress >= 1.0) {
                        ((Timer) (e.getSource())).stop();
                        progress = 1.0;
                    } else {
                        progress += 0.001;
                    }
                    cp = new CircleProgress(radius, 20, progress);
                    repaint();
                }
            });
            timer.start();
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(200, 200);
        }

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
            g2d.setRenderingHint(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
            g2d.setRenderingHint(RenderingHints.KEY_STROKE_CONTROL, RenderingHints.VALUE_STROKE_PURE);

            double x = (getWidth() - (radius * 2d)) / 2d;
            double y = (getHeight() - (radius * 2d)) / 2d;

            g2d.transform(AffineTransform.getTranslateInstance(x, y));

            g2d.setStroke(new BasicStroke(4f, BasicStroke.CAP_ROUND, BasicStroke.JOIN_ROUND));
            g2d.setColor(Color.DARK_GRAY);
            g2d.draw(cp);
            g2d.setColor(new Color(0, 161, 155));
            g2d.fill(cp);

            g2d.dispose();
        }

    }

}

CircularProgress.java

import java.awt.geom.Arc2D;
import java.awt.geom.Area;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Path2D;

public class CircleProgress extends Path2D.Double {

    private double radius;
    private double thickness; // subtracted from radius
    private double progress;

    public CircleProgress(double radius, double thickness, double progress) {
        this.radius = radius;
        this.thickness = thickness;
        this.progress = progress;
        build();
    }

    protected void build() {
        double innerRadius = radius - thickness;
        if (progress >= 1.0) {
            Area outter = new Area(new Ellipse2D.Double(0, 0, radius * 2d, radius * 2d));
            Area inner = new Area(new Ellipse2D.Double(thickness, thickness, innerRadius * 2d, innerRadius * 2d));
            outter.subtract(inner);
            append(outter, true);
        } else if (progress <= 0.0) {
            // What does this actually mean...?
        } else {
            double extent = -(360.0 * progress);
            double startAt = 90d;
            append(new Arc2D.Double(0, 0, radius * 2d, radius * 2d, startAt, extent, Arc2D.OPEN), true);
            append(new Arc2D.Double(thickness, thickness, innerRadius * 2d, innerRadius * 2d, startAt + extent, -extent, Arc2D.OPEN), true);
            closePath();
        }
    }

}