Code
JFrame.setDefaultLookAndFeelDecorated(true);
JFrame frame = new JFrame();
// com.sun.awt.AWTUtilities.setWindowOpacity(frame, .5f);
com.sun.awt.AWTUtilities.setWindowOpaque(frame, false); // 6u14
// 1.7.0 frame.setBackground(new Color(0, 0, 0, 0));
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
JPanel p = new JPanel();
p.add(new JButton("JButton"));
p.setBackground(new Color(.5f, .8f, .5f, .5f));
frame.getContentPane().add(p);
frame.setSize(320, 240);
frame.setLocationRelativeTo(null);
frame.setVisible(true);
References
