如何在自定义JComponent中使后台工作？

Question

在下面的示例中,我JComponent在绿色背景上绘制了自定义,但它没有出现.为什么会这样？

public class Test_Background {

    public static class JEllipse extends JComponent {

        private final Ellipse2D ellipse;

        public JEllipse(int width, int height) {
            ellipse = new Ellipse2D.Double(0, 0, width, height);

            setOpaque(true);
            setBackground(Color.GREEN);
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension((int) ellipse.getBounds().getMaxX(),
                                 (int) ellipse.getBounds().getMaxY());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);

            ((Graphics2D) g).draw(ellipse);
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                JEllipse e = new JEllipse(400, 300);

                JFrame f = new JFrame("Background Test");
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.add(e);
                f.pack();
                f.setVisible(true);
            }
        });
    }
}

Answer 1

JComponent不绘制其背景.您可以自己绘制它,也可以使用绘制背景的JPanel