Problems With Scroll

I have this problem with JScroll where no matter how I change the sizes, the scroll bar doesn't show up or it doesn't scroll when panels are added to the main panel. Btw, the mainPanel adds other panels too it, and when the amount of panels gets out of bounds, its suppose to scroll the panels. Heres the code

AddWorkout(){

//Panels.Dialg and Scroll

screen=new JDialog();

mainPanel=new JPanel();

scroll=new JScrollPane(mainPanel, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);

//Objects

frame=new FrameChooser();

image=new ImageLoader();

chooserFrame=new JFrame();

//Screen size

dim = Toolkit.getDefaultToolkit().getScreenSize();

//Layouts

mainPanel.setLayout(null);

screen.setLayout(null);

//methods

frame.newObject(AddWorkout.this);

setDatePanel();

//Sizes

screen.setSize(600,dim.height-20);

mainPanel.setSize(600,dim.height-20);

scroll.setSize(600,dim.height-20);

mainPanel.setPreferredSize(new Dimension(300, 300));

scroll.setPreferredSize(new Dimension(300,300));

//Stuff that you need to worry About

frame.repButton.addActionListener(this);

frame.cooldownButton.addActionListener(this);

frame.stretchButton.addActionListener(this);

frame.warmupButton.addActionListener(this);

frame.longRunButton.addActionListener(this);

frame.fartlekButton.addActionListener(this);

chooserFrame.getContentPane().add(frame.options);

chooserFrame.setSize(500,200);

chooserFrame.setLocation(600,0);

chooserFrame.setVisible(true);

//Last Step

screen.setVisible(true);

screen.getContentPane().add(scroll);

}

Please Help

[2674 byte] By [blackmagea] at [2007-11-27 9:19:05]