Thread: need a dropdown multiselection list

hi there.
Im creating a gui, and in it i need a component to display a list of checkboxes to a user in a dropdown fashion such that
a user can check any number of boxes before the component folds.

I have tried to create a component by adding check boxes into a Jpanel into a JScrollPane into another JPanel
which also contains a label as a title, a button for drop down and other two buttons (one for selecting all boxes and the other for selecting only checked boxes)

But the problem is, the JScrollpane doesnt get notified when the size of the panel containing checkboxes is larger than it can be viewed therefore scrollbars do not work

Here is my code

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Label;
import java.awt.Panel;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.List;
import java.util.Vector;
import javax.swing.JButton;
import javax.swing.JCheckBox;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
 
/**
 *
 * 
 */
public class Test extends JPanel{
 
    JScrollPane sb;
    JPanel panel;
    List itemCB;
    List items;
    JButton b,all,marked;
    int height = 25,width = 200;
    int cbWidth = 25;
    int xshift = 5,yshift = 5;
 
    String heading;
 
    public Test(String heading){
        this.heading = heading;
        setBackground(new Color(230,230,250));
        setLayout(null);
        items = new Vector();
        itemCB = new Vector();
        initComponent();
    }
    private void initComponent(){
        b = new JButton(){
            public void paint(Graphics g){
                super.paint(g);
                int x[] = {10,13,16};
                int y[] = {9,15,9};
                g.fillPolygon(x, y, 3);
            }
        };
        b.setSize(cbWidth, height);
        b.setLocation(width-cbWidth,0);
        b.addActionListener(new ActionListener(){
            private boolean clicked = false;
            public void actionPerformed(ActionEvent e){
                if(!clicked){
                    showItems();
                    clicked = true;
                }
                else{
                    setSize(width,height);
                    clicked = false;
                }
            }
        });
        add(b);
 
        Label l = new Label(heading){
            public void paint(Graphics g){
                g.setColor(Color.lightGray);
                g.drawRect(0,0, getWidth()-1,getHeight()-1);
            }
        };
        l.setSize(width-cbWidth,height);
        l.setLocation(0,0);
        add(l);
 
        sb = new JScrollPane();
        sb.setSize(width,height*(items.size()+1));
        sb.setLocation(1, height+1);
        sb.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
 
        add(sb);
 
        all = new JButton("All");
        all.setSize(50, height);
        all.setLocation(width-55,getHeight()-30);
        all.addActionListener(new ActionListener(){
            boolean selected = false;
            public void actionPerformed(ActionEvent e){
                b.getActionListeners()[0].actionPerformed(e);
                if(selected){
                    for(int i=0;i<itemCB.size();i++)
                        ((JCheckBox)itemCB.get(i)).setSelected(false);
                    selected = false;
                }
                else{
                    for(int i=0;i<itemCB.size();i++)
                        ((JCheckBox)itemCB.get(i)).setSelected(true);
                    selected = true;
                }
            }
        });
        add(all);
 
        marked = new JButton("Marked");
        marked.setSize(80, height);
        marked.setLocation(width-150,getHeight()-30);
        marked.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e){
                b.getActionListeners()[0].actionPerformed(e);
                getSelectedItems();
            }
        });
        add(marked);
 
        setSize(width,height);
    }
 
    public void addItem(String text){
        items.add(text);
        final JCheckBox cb = new JCheckBox(text);
        cb.setSize(width,height);
        cb.setLocation(0,height*(items.size()-1));
        cb.addMouseListener(new MouseAdapter(){
            boolean clicked = false;
            @Override
            public void mouseEntered(MouseEvent m){
                cb.setBackground(Color.lightGray);
            }
            @Override
            public void mouseExited(MouseEvent m){
                cb.setBackground(new Color(240,240,240));
 
            }
        });
        itemCB.add(cb);
    }
 
    private void showItems(){
        panel = new JPanel();
        panel.setLayout(null);
 
        for(int i=0;i<itemCB.size();i++){
            panel.add((JCheckBox)itemCB.get(i));
        }
 
 
        panel.setSize(width - 2,height*(items.size()+1)-height);
        if(items.size()<9){
            sb.setSize(width-2,height*(items.size()+1)-height);
            setSize(width,height*(items.size()+1)+height+yshift*2);
        }
        else{
            sb.setSize(width-2,height*(9)-height);
            setSize(width,height*(9)+height+yshift*2);
        }
        sb.setViewportView(panel);
        marked.setLocation(width-140,getHeight()-30);
        all.setLocation(width-55,getHeight()-30);
    }
 
    public void setMarkedButtonActionListener(ActionListener al){
        marked.addActionListener(al);
    }
 
    public void setAllButtonActionListener(ActionListener al){
        all.addActionListener(al);
    }
 
    public List getSelectedItems(){
        List selectedItems = new Vector();
        for(int i=0;i<itemCB.size();i++){
            if(((JCheckBox)itemCB.get(i)).isSelected()){
                selectedItems.add(items.get(i));
            }
        }
        if(selectedItems.isEmpty())
            return null;
        return selectedItems;
    }
 
    public List getAllItems(){
        return items;
    }
 
    @Override
    public void paint(Graphics g){
        super.paint(g);
        g.drawRect(0,height, getWidth()-1,getHeight()-height-1);
    }
    public static void main(String ar[]){
        JFrame f = new JFrame();
        f.setLayout(null);
        f.setSize(500,500);
        f.setLocation(400,200);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Test sp = new Test("");
        sp.setLocation(20, 100);
        for(int i=0;i<10;i++)
        sp.addItem("Item "+(i+1));
        f.add(sp);
        f.setVisible(true);
    }
}

Any help please!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

This suggestion is pretty cool, but it is dated. It may also require tweaking to not fold up every time a selection is made, but it's still a good starting point.

i tried the pretty cool suggestion of yours before I wrote the code I posted.
The problem is I coulnt find out what listener of the ComboBox to deal with so that I could stop it from folding upon single selection.

fixed it.
just a layout problem.
instead of:

panel.setLayout(null);

I put:

panel.setLayout(new BoxLayout(panel,1));

also added the maximum size of the checkboxes.
there I got my dropdown multiselection list working.
thanks @ GregBrannon.