code: 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;
      }
docs: If the specified key is not already associated with a value (or is mapped
  to {@code null}), attempts to compute its value using the given mapping
  function and enters it into this map unless {@code null}.
  
  @param keyPartA        for the key
  @param keyPartB        for the key
  @param mappingFunction creates values based upon keys if the key pair is missing
  @return the newly created or stored value.