code: public int add (K key, int amount)
      {
          CountEntry<K> entry = _backing.get(key);
          if (entry == null) {
              _backing.put(key, new CountEntry<K>(key, amount));
              return amount;
          }
          return (entry.count += amount);
      }
docs: Add the specified amount to the count for the specified key, return the new count.
  Adding 0 will ensure that a Map.Entry is created for the specified key.