code: public void add(T value) {
          if (value == null)
              return;
  
          if (map.containsKey(value)) {
              map.put(value, map.get(value) + 1);
          } else {
              map.put(value, 1l);
          }
      }
docs: Adds an occurence of this value into the Histogram
  
  @param value