Java – jscrollpane to scrolling a panel

i have to writing an applet, in left side i must use an panel to contain a list of vehicles that can be a list of buttons,what is the problem, number of the vehicles are not given !!!
so, i need to scrolling panel when number of vehicles is too much,

i do this for jframe, but it didn't work correct with panel, please help me with an example

the code i use to scrolling panel is :

 public class VehicleList extends JPanel {
    private ArrayList<VehicleReport> vehicles;
    private ArrayList<JButton> v_buttons =  new ArrayList<JButton>();



 public void showList(ArrayList<Vehicles> vehicles)
 {
   this.vehicles = vehicles;
   //...
    add(getScrollpane());
    setSize(155,300);

 }

public JScrollPane getScrollpane()
{
  JPanel panel = new JPanel();
  panel.setPreferredSize(new DimensionUIResource(150, 300));
   GridBagLayout gridbag = new GridBagLayout();
    GridBagConstraints constraint = new GridBagConstraints();
    panel.setLayout(gridbag);
    constraint.fill = GridBagConstraints.HORIZONTAL;
    JLabel title = new JLabel("Vehiles list");
    constraint.gridwidth = 2;
    constraint.gridx = 0;
    constraint.gridy = 0;
    constraint.ipady = 230;
    gridbag.setConstraints(title, constraint);
    panel.add(title);
    // end of set title
    constraint.gridwidth = 1;
    int i=1;

    for(JButton jb : v_buttons )
    {
        constraint.gridx =0;
        constraint.gridy = i;
        gridbag.setConstraints(jb, constraint);
        panel.add(jb);
        JLabel vehicle_lable = new JLabel("car" + i);
        constraint.gridx = 1;
        constraint.gridy = i;
        gridbag.setConstraints(vehicle_lable, constraint);
        panel.add(vehicle_lable);
        i++;
    }
JScrollPane jsp = new JScrollPane(panel);
 return jsp;

}

}

in jaframe after add jscrollpane to jframe i place this

pack();

setSize(250, 250);

setLocation(100, 300);

and it work clearly!!!!