【Java】panel.getGraphics()报错空指针异常的解决方法

如下的Swing代码：

public class GraphicsNullPointerDemo {

    public static void main(String[] args) {
        JFrame frame = new JFrame("空指针异常");
        frame.setLayout(new FlowLayout());
        frame.setSize(400, 200);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel panel = new JPanel();
        frame.add(panel, BorderLayout.CENTER);
        System.out.println(panel.getGraphics());
        frame.setVisible(true);
    }

}

输出的panel.getGraphics()值是null

反之，调换frame.setVisible(true)和panel.getGraphics()的顺序，则不会为null

事实上，getGraphics()经常有空指针异常的可能性，所谓"This method will return null if this component is currently not displayable."，因此应该先setVisible(true)后getGraphics()