// connect to category manager for this project area
                CategoriesManager catManager = CategoriesManager.createInstance(auditableClient, projectarea_handle, null);
                // get the root node
                CategoryTreeNode root = catManager.getRoot();
                // loop thru the roots children
                 for (CategoryTreeNode nodei : root.getChildren())
                 {
                    
                 }

note that this is a tree of categories, not a flat list..  so for each node, you need to check if there are children.