docs: public int computeIfAbsent(final int key, final IntUnaryOperator mappingFunction)
      {
          int value = get(key);
          if (value == missingValue)
          {
              value = mappingFunction.applyAsInt(key);
              if (value != missingValue)
              {
                  put(key, value);
              }
          }
  
          return value;
      }