code: public void update(final Memory srcMem, final ArrayOfItemsSerDe<T> serDe) {
      final ItemsSketch<T> that = ItemsSketch.getInstance(srcMem, comparator_, serDe);
      gadget_ = updateLogic(maxK_, comparator_, gadget_, that);
    }
docs: Iterative union operation, which means this method can be repeatedly called.
  Merges the given Memory image of a ItemsSketch into this union object.
  The given Memory object is not modified and a link to it is not retained.
  It is required that the ratio of the two K values be a power of 2.
  This is easily satisfied if each of the K values is already a power of 2.
  If the given sketch is null or empty it is ignored.
  
  <p>It is required that the results of the union operation, which can be obtained at any time,
  is obtained from {@link #getResult() }.</p>
  @param srcMem Memory image of sketch to be merged
  @param serDe an instance of ArrayOfItemsSerDe