在Ubuntu中安装Memsql和MySql(Memsql and MySql installation in Ubuntu)

 你可以使用数学的魔力特别是三角学来计算你的线在圆上的终点。 但最简单的方法是使用仿射变换。 看看这个paint()方法 
 
public void paint (Graphics g){
    super.paint(g);    
    g.setColor(Color.BLACK);        
    Graphics2D gg = (Graphics2D) g.create();   // create new layer and cast to advanced Graphics2d
    gg.rotate(Math.toRadians(x), 90,90);       // rotate new layer at center position 90,90 around x degrees 
    gg.drawLine(90,90,0,0);                    // draw line to center position
    gg.dispose();                              // push new layer back
}

You can use the magic of math particularly trigonometry to calc the end points of your line being on a circle. But the simplest way would be to use affine transformation. Have a look at this paint() method
 
public void paint (Graphics g){
    super.paint(g);    
    g.setColor(Color.BLACK);        
    Graphics2D gg = (Graphics2D) g.create();   // create new layer and cast to advanced Graphics2d
    gg.rotate(Math.toRadians(x), 90,90);       // rotate new layer at center position 90,90 around x degrees 
    gg.drawLine(90,90,0,0);                    // draw line to center position
    gg.dispose();                              // push new layer back
}