void repaint()
{
Graphics g = canvas.getGraphics();// 绘图对象
g.setColor(Color.blue);
g.fillRect(0, 0, canvaswidth, canvasheight); // 在（0,0）画一个x，y大的矩形
drawline(); //绘制方格
}
private void drawline() {
Graphics g = canvas.getGraphics();// 绘图对象
g.setColor(Color.black);
for (int i = 0; i < 20; i++) {
g.drawLine(i, 0, i, canvasheight);
g.drawLine(0, i, canvaswidth, i);
}
//for (int j = 0; j < canvasheight; j+=10) {
//}
}