code: private String getNeighborGroup(Tile tile, Tile neighbor)
      {
          final String neighborGroup;
          if (isTransition(neighbor))
          {
              if (isTransition(tile))
              {
                  neighborGroup = getOtherGroup(tile, neighbor);
              }
              else
              {
                  neighborGroup = null;
              }
          }
          else
          {
              neighborGroup = mapGroup.getGroup(neighbor);
          }
          return neighborGroup;
      }
docs: Get the neighbor group depending if it is a transition tile.
  
  @param tile The current tile.
  @param neighbor The neighbor tile.
  @return The neighbor group, <code>null</code> if none.