using frame.setBackground(0, 255, 0, 0) doesn'

Question

so I have been make a JFrame background transparent by setting the background alpha to 0, however if I run the program in linux the JFrame background is white why is this?

ok I found its got something to do with rendering graphics to the frame, this method is old I don't need it in the program anymore but i would still like to know why this happens (its just in Linux works find in windows)

here is a runnable example

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

class Main extends JFrame{
private void init() {

    this.setPreferredSize(new Dimension(1420, 820));
    this.setUndecorated(true);
    this.setResizable(false);
    this.setDefaultCloseOperation(this.EXIT_ON_CLOSE);
    this.setBackground(new Color(0, 255, 0, 0));
    this.requestFocus();
    this.setVisible(true);
    this.validate();
    this.pack();
    Color color = UIManager.getColor("activeCaptionBorder");
    this.getRootPane().setBorder(BorderFactory.createLineBorder(color, 1));
    paintInfo();
}

private void paintInfo() {
    Graphics g = this.getGraphics();
    g.setColor(new Color(222, 222, 222, 4));
    g.setFont(new Font("Arial Black", Font.BOLD, 15));
    g.setColor(Color.BLACK);
    g.drawString("test String ",this.getWidth()/2, this.getHeight()/2);
    g.dispose();
}

public static void main(String[] args) {
    JFrame.setDefaultLookAndFeelDecorated(true);
    new Main().init();
}
}

Answer 1

Let's start with...

private void paintInfo() {
    Graphics g = this.getGraphics();
    g.setColor(new Color(222, 222, 222, 4));
    g.setFont(new Font("Arial Black", Font.BOLD, 15));
    g.setColor(Color.BLACK);
    g.drawString("test String ",this.getWidth()/2, this.getHeight()/2);
    g.dispose();
}

Isn't how painting is done in Swing and disposing of a Graphics context you didn't create will lead to manner of strange things...

Instead, create your frame, set up it's transparency and then add another component to it, which does the actual painting that you want, for example...

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.GridBagLayout;
import java.awt.LinearGradientPaint;
import java.awt.Point;
import java.awt.Rectangle;
import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

/**
 *
 * @author swhitehead
 */
public class JavaApplication233 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        GraphicsEnvironment ge
                        = GraphicsEnvironment.getLocalGraphicsEnvironment();
        GraphicsDevice gd = ge.getDefaultScreenDevice();

        //If translucent windows aren't supported, exit.
        if (!gd.isWindowTranslucencySupported(GraphicsDevice.WindowTranslucency.PERPIXEL_TRANSLUCENT)) {
            System.err.println("Per-pixel translucency is not supported");
            System.exit(0);
        } else {
            new JavaApplication233();
        }
    }

    public JavaApplication233() {
        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.setUndecorated(true);
                frame.setBackground(new Color(0, 0, 0, 0));
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        public TestPane() {
            setOpaque(false);
            setLayout(new GridBagLayout());
            add(new JLabel("I'm not wearing anything"));
//          Color color = UIManager.getColor("activeCaptionBorder");
            setBorder(BorderFactory.createLineBorder(Color.BLACK, 1));
        }

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

        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            LinearGradientPaint lgp = new LinearGradientPaint(
                            new Point(0, 0),
                            new Point(0, getHeight()),
                            new float[]{0f, 1f},
                            new Color[]{applyAlpha(Color.RED), applyAlpha(Color.YELLOW)}
            );
            g2d.setPaint(lgp);
            g2d.fill(new Rectangle(0, 0, getWidth(), getHeight()));
            g2d.dispose();
        }

        protected Color applyAlpha(Color color) {
            return new Color(color.getRed(), color.getGreen(), color.getBlue(), 64);
        }

    }

}