swing开发，让面板居中显示

        int width = this.getContentPane().getWidth();
        int height = this.getContentPane().getHeight();
        int screenwidth = this.getToolkit().getDefaultToolkit().getScreenSize().width;
        int screenheight = this.getToolkit().getDefaultToolkit().getScreenSize().height;

        int x = screenwidth/2 - width/2;
        int y = screenheight/2 - height/2;