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()
