code: private static void expandChildren(BaseComponent parent, int depth) {
          if (parent == null || depth <= 0) {
              return;
          }
  
          depth--;
  
          for (Object object : parent.getChildren()) {
              Treenode item = (Treenode) object;
              item.setCollapsed(depth <= 0);
              expandChildren(item, depth);
          }
      }
docs: Expand / collapse tree children to specified depth.
  
  @param parent Tree children whose tree items are to be expanded / collapsed.
  @param depth Expand tree items to this depth. Tree items below this depth are collapsed.