<zk>
    <window apply="MyComposer" border="normal" title="TestWindow" mode="modal">
        <tree id="tstTree" mold="paging" pageSize="5">
            <template name="model">
                <treeitem label="${each}" />
            </template>
        </tree>
    </window>
</zk>

import java.util.Arrays;

import org.zkoss.zk.ui.Component;
import org.zkoss.zk.ui.select.annotation.Wire;
import org.zkoss.zk.ui.util.GenericForwardComposer;
import org.zkoss.zul.DefaultTreeModel;
import org.zkoss.zul.DefaultTreeNode;
import org.zkoss.zul.Tree;

public class MyComposer extends GenericForwardComposer<Component> {

    @Wire
    private Tree tstTree;

    @Override
    public void doAfterCompose(Component comp) throws Exception {
        super.doAfterCompose(comp);
        tstTree.setModel(createNewModel());
    }

    private static DefaultTreeModel<String> createNewModel() {
        return new DefaultTreeModel<>(
            node("root",
                node("item-0",
                    node("item-0-0"),
                    node("item-0-1"),
                    node("item-0-2", node("item-0-2-0"), node("item-0-2-1")),
                    node("item-0-3", node("item-0-3-0"), node("item-0-3-1"), node("item-0-3-2")),
                    node("item-0-4"),
                    node("item-0-5"),
                    node("item-0-6", node("item-0-6-0"), node("item-0-6-1")),
                    node("item-0-7")
                )
            )
        );
    }

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