docs: public V computeIfAbsent(final int keyPartA, final int keyPartB, final EntryFunction<? extends V> mappingFunction)
      {
          V value = get(keyPartA, keyPartB);
          if (value == null)
          {
              value = mappingFunction.apply(keyPartA, keyPartB);
              if (value != null)
              {
                  put(keyPartA, keyPartB, value);
              }
          }
  
          return value;
      }