import javax.swing.*;

public class TransparencyTest extends JFrame {
  public static void main(String[] args) {
    new TransparencyTest("Test");
  }
  public TransparencyTest(String title) {
    super(title);
    setUndecorated(true);
    JPanel panel = new JPanel();
    panel.add(new JLabel("Test Text"));
    panel.setOpaque(false);
    setContentPane(panel);
    getLayeredPane().setOpaque(false);
    getRootPane().setOpaque(false);
    setBackground(new java.awt.Color(0, 0, 0, 0));
    pack();
    setLocation(500, 500);
    setVisible(true);
  }
}