Oracle's "Translucent and Shaped Windows", slightly changed

/**
 * ORIGINAL CODE BY ORACLE:
 * <p>
 * https://docs.oracle.com/javase/tutorial/uiswing/examples/misc/GradientTranslucentWindowDemoProject/src/misc/GradientTranslucentWindowDemo.java
 * <p>
 * THIS CODE HAS BEEN SLIGHTLY CHANGED TO TEACH MORE WITH LESS, AND TO BE MORE UP-TO-DATE.
 * <p>
 * <p>
 * <p>
 * <p>
 * <p>
 * <p>
 * <p>
 * <p>
 * <p>
 * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
 * <p>
 * Redistribution and use in source and binary forms, with or without modification, are permitted provided that the
 * following conditions are met:
 * <p>
 * - Redistributions of source code must retain the above copyright notice, this list of conditions and the following
 * disclaimer.
 * <p>
 * - Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following
 * disclaimer in the documentation and/or other materials provided with the distribution.
 * <p>
 * - Neither the name of Oracle or the names of its contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 * <p>
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

import com.sun.awt.AWTUtilities;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;
import java.awt.geom.Ellipse2D;

import static java.awt.GraphicsDevice.WindowTranslucency;




public class TranslucentWindowTest extends JFrame {


    public TranslucentWindowTest() {

        super("GradientTranslucentWindow");

        //        setUndecorated(true);

        setBackground(new Color(0, 0, 0, 0));
        setSize(new Dimension(300, 200));
        setLocationRelativeTo(null);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        JPanel panel = new JPanel() {

            @Override
            protected void paintComponent(Graphics g) {

                if (g instanceof Graphics2D) {
                    final int R = 240;
                    final int G = 240;
                    final int B = 240;

                    Paint p = new GradientPaint(0.0f, 0.0f, new Color(R, G, B, 0), 0.0f, getHeight(), new Color(R, G, B, 255), true);
                    Graphics2D g2d = (Graphics2D) g;
                    g2d.setPaint(p);
                    g2d.fillRect(0, 0, getWidth(), getHeight());
                }
            }
        };
        setContentPane(panel);
        setLayout(new GridBagLayout());
        add(new JButton("I am a Button"));

        addComponentListener(new ComponentAdapter() {

            @Override
            public void componentResized(ComponentEvent evt) {

                Shape shape = new Ellipse2D.Float(0, 0, getWidth(), getHeight());
                AWTUtilities.setWindowShape(TranslucentWindowTest.this, shape);
            }
        });
    }


    public static void main(String[] args) {

        // Determine what the GraphicsDevice can support.
        final GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
        final GraphicsDevice gd = ge.getDefaultScreenDevice();

        if (!gd.isWindowTranslucencySupported(WindowTranslucency.PERPIXEL_TRANSLUCENT)) {
            System.err.println("Per-pixel translucency is not supported");
            System.exit(0);
        }

        if (!gd.isWindowTranslucencySupported(WindowTranslucency.PERPIXEL_TRANSPARENT)) {
            System.err.println("Shaped windows are not supported");
            System.exit(0);
        }

        JFrame.setDefaultLookAndFeelDecorated(true); // EITHER DO THIS - OR UNCOMMENT "setUndecorated(true);" FURTHER UP

        SwingUtilities.invokeLater(() -> new TranslucentWindowTest().setVisible(true));
    }
}