package tst;

import java.util.Arrays;

import org.zkoss.zul.DefaultTreeModel;
import org.zkoss.zul.DefaultTreeNode;

public class MyModel {

    public DefaultTreeModel<String> getTreeModel() {
        return new DefaultTreeModel<>(node("root",
                node("item1", node("item11", node("item111"), node("item112"))),
                node("item2", node("item21", node("item211")), node("item22", node("item221")))
        ));
    }

    @SafeVarargs
    private static DefaultTreeNode<String> node(String name, DefaultTreeNode<String>... children) {
        return (children.length == 0) ? new DefaultTreeNode<>(name) : new DefaultTreeNode<>(name, Arrays.asList(children));
    }
}

<zk>
    <window viewModel="@id('vm') @init('tst.MyModel')">
        <tree model="@init(vm.treeModel)" onSelect="alert(event.getSelectedObjects());">
            <template name="model">
                <treeitem label="${each}" checkable="false" />
            </template>
        </tree>
    </window>
</zk>