class test extends java.awt.Frame
{
java.awt.Panel panel;
public test()
{
setSize(320,240);
panel=new java.awt.Panel();
panel.getGraphics().drawLine(0,0,120,120);
add(panel);
setVisible(true);
}
public static void main(String[] args)
{
new test();
}
}
panel.getGraphics().drawLine(0,0,120,120);
add(panel);
setVisible(true);new的时候并没有真正的创建实际的Panel,只有在setVisible之后才notify,通过本地同位体包创建Panel,所以在setVisible之前,panel.getGraphics() = null