code: public void addNewItem(M newItem) {
      if (this.items == null) {
        this.items = createEmptyList();
      }
  
      this.items.add(newItem);
    }
docs: <p>
  Add a single item.
  </p>
  
  <p>
  If {@link #items} is null an empty {@link List} will be created first.
  </p>
  
  @param newItem item to add