When sorting a list model that is bound to a Listbox component that does not have a Listhead child, Listbox.onListDataChange(...) throws a NullPointerException.

Consider this simple view model:

package org.zkoss.testing;

import java.util.ArrayList;
import java.util.Comparator;

import org.zkoss.bind.annotation.Command;
import org.zkoss.bind.annotation.Init;
import org.zkoss.zkplus.databind.BindingListModelList;
import org.zkoss.zul.ListModelList;

public class SimpleViewModel {

private static final Comparator<String> COMPARATOR = new Comparator<String>() {
@Override
public int compare(String o1, String o2)

{ return o1.compareTo(o2); }

};

private ListModelList<String> listModel;

@Init
public void init()

{ listModel = new BindingListModelList<String>(new ArrayList<String>(), false); listModel.add("ONE"); listModel.add("TWO"); listModel.add("THREE"); }

public ListModelList<String> getListModel()

{ return listModel; }

public void setListModel(ListModelList<String> listModel)

{ this.listModel = listModel; }

@Command
public void sortList()

{ listModel.sort(COMPARATOR, true); }

}

Bound to this ZUL:

<vbox apply="org.zkoss.bind.BindComposer" viewModel="@id('vm') @init('org.zkoss.testing.SimpleViewModel')">

<button label="Sort Values" onClick="@command('sortList')"/>

<listbox id="listbox" model="@bind(vm.listModel)">
<template name="listboxModel">
<listitem><listcell value="@bind(each)"/></listitem>
</template>
</listbox>

</vbox>

When the sortList() command is executed, the listbox throws a NullPointerException:

Apr 8, 2013 2:27:35 PM org.zkoss.bind.impl.BinderImpl$CommandEventListener onEvent:1327
SEVERE: null
java.lang.NullPointerException
at org.zkoss.zul.Listbox.onListDataChange(Listbox.java:2553)
at org.zkoss.zul.Listbox.access$800(Listbox.java:261)
at org.zkoss.zul.Listbox$4.onChange(Listbox.java:461)
at org.zkoss.zul.AbstractListModel.fireEvent(AbstractListModel.java:59)
at org.zkoss.zul.ListModelList.sort(ListModelList.java:417)
at org.zkoss.testing.SimpleViewModel.sortList(SimpleViewModel.java:40)
...
[ Stack trace truncated ]

The fix seems as easy as just adding a null-pointer check to the enclosing conditional in Listbox.onListDataChange at line 2550:

if (event.getType() == ListDataEvent.STRUCTURE_CHANGED
&& _model instanceof Sortable) {

Changes to:

if (event.getType() == ListDataEvent.STRUCTURE_CHANGED
&& _model instanceof Sortable
&& _listhead != null) {

A workaround is to add the Listhead component with visibility set to false.