code: public VectorTile addTile(TileCode tileCode) {
  		String code = tileCode.toString();
  		VectorTile tile = tiles.get(code);
  		if (tile == null) {
  			tile = new VectorTile(tileCode, calcBoundsForTileCode(tileCode), this);
  			tile.setPictureStyle(new PictureStyle(layer.getOpacity()));
  			tiles.put(code, tile);
  		}
  		return tile;
  	}
docs: Adds the tile with the specified code to the cache or returns the tile if it's already in the cache.
  
  @param tileCode
  A {@link TileCode} instance.