code: public V computeIfAbsent(final int key, final IntFunction<? extends V> mappingFunction)
      {
          V value = getMapped(key);
          if (value == null)
          {
              value = mappingFunction.apply(key);
              if (value != null)
              {
                  put(key, value);
              }
          }
          else
          {
              value = unmapNullValue(value);
          }
  
          return value;
      }
docs: Get a value for a given key, or if it does not exist then default the value via a
  {@link java.util.function.IntFunction} and put it in the map.
  <p>
  Primitive specialized version of {@link java.util.Map#computeIfAbsent}.
  
  @param key             to search on.
  @param mappingFunction to provide a value if the get returns null.
  @return the value if found otherwise the default.