Is this an issue with checkbox or some tweak!

package j1vilk89$v13;

import java.util.*;
import org.zkoss.zk.ui.*;
import org.zkoss.zk.ui.event.*;
import org.zkoss.zk.ui.util.*;
import org.zkoss.zk.ui.ext.*;
import org.zkoss.zk.au.*;
import org.zkoss.zk.au.out.*;
import org.zkoss.zul.*;
public class TestComposer extends GenericForwardComposer{	
 
	private List<Product> clinicalSites;
	private List<Checkbox> selectedCheckboxs;
	private Label msg;
 
	private Grid mgrid;
 
	@Override
	public void doAfterCompose(Component comp) throws Exception {
		super.doAfterCompose(comp);
		selectedCheckboxs = new ArrayList<Checkbox>();
 
		clinicalSites = new ArrayList<Product>();
		int num = 10;		
		for (int i = 0; i < num; i++) {
			clinicalSites.add(new Product("item" + i));			
		}		
	}
	public void onModifySelectedList(ForwardEvent event){
		Checkbox checkbox = (Checkbox) event.getOrigin().getTarget();		
		if (checkbox.isChecked()) 
			selectedCheckboxs.add(checkbox);
		else selectedCheckboxs.remove(checkbox);
	}
	public void onClick$updateBtn() {		
		StringBuilder sb = new StringBuilder();
		for (Checkbox checkbox : selectedCheckboxs) {
			sb.append(checkbox.getLabel());
			sb.append(", ");
		}
		if (selectedCheckboxs.size() > 0){
			int length = sb.length();
			sb.replace(length - 2, length, "");
		}
		msg.setValue(sb.toString());		
	}	
 
	public List<Product> getAllClinicalSites(){
		return clinicalSites;		
	}
	public class Product{
		private String _name;
		public Product(String name) {
			super();
			this._name = name;
		}
		public void setName(String name) {
			this._name = name;
		}	
 
		public String getName() {
			return _name;
		}		
	}
 
  public void onCheck$selectedAllCheckBox(CheckEvent e){
    List components = mgrid.getRows().getChildren();
    for(Object obj:components){
      Row comp = (Row) obj;
      Checkbox ck = (Checkbox) comp.getChildren().get(0);
      ck.setChecked(e.isChecked());
   }
  }
}

<?init class="org.zkoss.zkplus.databind.AnnotateDataBinderInit" ?>
<zk>
        <window id="win" apply="j1vilk89$v13.TestComposer">
                <listbox mold="paging" pageSize="10" checkmark="true" multiple="true">
                      <listhead>
                              <listheader label=""></listheader>
                              <listheader label=""></listheader>
                      </listhead>
                      <listitem>
                              <listcell>
                                      <textbox mold="rounded"></textbox>
                              </listcell>
 
                              <listcell>
                                      <checkbox></checkbox>
                              </listcell>
 
                      </listitem>
              </listbox>
	<button id="updateBtn" label="Update" />
	<label id="msg"/>
        </window>
</zk>