为什么在JPanel上加滚动条不成功啊，滚动条不能滚动

[Kittysimida]

void add()
{
CookiePanel rightpanel=new CookiePanel();
JScrollPane scroll = new JScrollPane();
scroll.setBounds(200, 120, 770, 590);
scroll.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
scroll.setBackground(null);
scroll.setOpaque(false);
scroll.getViewport().setOpaque(false);
scroll.setBorder(null);
this.add(scroll);
scroll.add(rightpanel);
}

public class CookiePanel extends JPanel{
public CookiePanel()
{

this.setBounds(0,0,800,990);
//this.setBounds(200, 120, 800, 550);
this.setBackground(null);
this.setOpaque(false);
this.setPreferredSize(null);
//this.setLayout(new FlowLayout(FlowLayout.LEFT,30,20));
this.setLayout(null);
}
}

[Inhibitory]

JScrollPane scroll = new JScrollPane(rightpanel);

[hike1992]

scroll.add(rightpanel); 把这句改成 scroll.setViewportView(rightpanel); JScrollPanel是不能用.add(),加组件的