code: 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;
      }
docs: Get a value for a given key, or if it does ot exist then default the value via a {@link IntFunction}
  and put it in the map.
  
  @param key             to search on.
  @param mappingFunction to provide a value if the get returns null.
  @return the value if found otherwise the default.