### Question:
DecisionNavigatorTreeView implements DecisionNavigatorTreePresenter.View { @Override public void clean() { RemoveHelper.removeChildren(items); } @Inject  DecisionNavigatorTreeView(final HTMLDivElement view,
                                     final HTMLDivElement items,
                                     final ManagedInstance<TreeItem> managedInstance,
                                     final Elemental2DomUtil util); @Override void init(final DecisionNavigatorTreePresenter presenter); @Override HTMLElement getElement(); @Override void clean(); @Override void setup(final List<DecisionNavigatorItem> items); @Override void select(final String uuid); @Override void deselect();  }### Answer:
@Test public void testClean() { final Element element = mock(Element.class); items.firstChild = element; when(items.removeChild(element)).then(a -> { items.firstChild = null; return element; }); treeView.clean(); verify(items).removeChild(element); }