Adding a JPanel to a JScrollPane?

Question

I have a JPanel which may hold a lot of components, this may run off the end of the window so I want to add the components to a JPanel and the JPanel to a JScrollPane to deal with this can anyone tell me why the below code won't do this?
import javax.swing.JCheckBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextField;
import javax.swing.ScrollPaneConstants;
public class Test extends JScrollPane
     public JFrame testFrame;
     public JPanel testPanel;
     public JCheckBox checkBox;
     public static void main(String[] args)
          new Test();
     public Test()
          testFrame = new JFrame("test");
          //testFrame.getContentPane().setLayout(null);
          testPanel = new JPanel();          
          JScrollPane sp = new JScrollPane(testPanel);
          //add the panel to the scroll pane
          //sp.setViewportView(testPanel);
          //sp.getVerticalScrollBar();
          this.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED);
          this.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED);
          testFrame.getContentPane().add(sp);
          //add the scroll pane to the frame
         JLabel lbl1 = new JLabel("blah");
         testPanel.add(lbl1);
        testFrame.getContentPane().add(testPanel);
         for(int i=0; i==20; i++)
               checkBox = new JCheckBox(String.valueOf(i));
              //checkBox.addItemListener(this);
              JTextField txt1 = new JTextField(10);
              txt1.setText("hello world");
             testPanel.add(checkBox);
            testPanel.add(txt1);
        testPanel.validate();
        testPanel.setVisible(true);
        testFrame.pack();
        testFrame.setVisible(true);
        testFrame.setSize(100, 200);
        testFrame.show();
}

Answer 1

Actually not extending JScrollPane means that
this.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED);
this.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED);
raised compile errors so you've clearly not bothered to run it.
Anyway, I figured out the solution, for anyone who may find it useful here it is:
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextField;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
public class Test
     public JFrame testFrame;
     public JPanel testPanel;
     public JCheckBox checkBox;
     public JButton btnDone;
     public static void main(String[] args)
          new Test();
     public Test()
          testFrame = new JFrame("test");
          testPanel = new JPanel();
          testPanel.setLayout(new GridBagLayout());
          JScrollPane sp = new JScrollPane();
          GridBagConstraints gridBagConstraints = new GridBagConstraints();
         JLabel lbl1 = new JLabel("blah");
         testPanel.add(lbl1);
         for(int i=0; i<20; i++)
               checkBox = new JCheckBox(String.valueOf(i));
              //checkBox.addItemListener(this);
              JTextField txt1 = new JTextField(10);
              txt1.setText("hello world");
             gridBagConstraints.gridx = 0;
             gridBagConstraints.gridy = i+1;
             testPanel.add(checkBox, gridBagConstraints);
             gridBagConstraints.gridx = 1;
             gridBagConstraints.gridy = i+1;
             testPanel.add(txt1, gridBagConstraints);
        gridBagConstraints.gridx = 1;
        gridBagConstraints.gridy = gridBagConstraints.gridy+1;
        testPanel.add(btnDone = new JButton("Done"), gridBagConstraints);
          sp.setViewportView(testPanel);
         testFrame.getContentPane().add(sp);
         //btnDone.addActionListener(this);
        testPanel.validate();
        testFrame.pack();
        testFrame.setVisible(true);
        testFrame.setSize(200, 300);
        testFrame.show();
}