更改JLabel组件的背景

我使用的代码是:

public class Test extends JFrame implements ActionListener {

    private static final Color TRANSP_WHITE =
        new Color(new Float(1), new Float(1), new Float(1), new Float(0.5));
    private static final Color TRANSP_RED =
        new Color(new Float(1), new Float(0), new Float(0), new Float(0.1));
    private static final Color[] COLORS =
        new Color[]{TRANSP_RED, TRANSP_WHITE};
    private int index = 0;
    private JLabel label;
    private JButton button;

    public Test() {
        super();

        setLayout(new BoxLayout(getContentPane(), BoxLayout.Y_AXIS));
        label = new JLabel("hello world");
        label.setOpaque(true);
        label.setBackground(TRANSP_WHITE);

        getContentPane().add(label);

        button = new JButton("Click Me");
        button.addActionListener(this);

        getContentPane().add(button);

        pack();
        setVisible(true);
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (e.getSource().equals(button)) {
            label.setBackground(COLORS[index % (COLORS.length)]);
            index++;
        }
    }

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

之前: