为什么Canvas.getGraphics()获取不到呢解决方案

为什么Canvas.getGraphics()获取不到呢
用的是jdk1.7，我是java新手，
panel=new Panel();

Canvas canvas=new Canvas();
canvas.setSize(500, 500);
canvas.setBackground(Color.black);

canvas.setVisible(true);
canvas.setEnabled(true);
panel.add(canvas);
panel.setLayout(null);
Graphics gp = canvas.getGraphics();
gp.drawArc(30, 30, 30, 30, 30, 30);

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文明一点