docs: public V computeIfAbsent(final int key, final IntFunction<? extends V> mappingFunction) {
          checkNotNull(mappingFunction, "mappingFunction cannot be null");
          V value = get(key);
          if (value == null) {
              value = mappingFunction.apply(key);
              if (value != null) {
                  put(key, value);
              }
          }
          return value;
      }