public void setUnit(final String UNIT) {
        if (null == unit) {
            _unit = UNIT;
            fireTileEvent(VISIBILITY_EVENT);
            fireTileEvent(REDRAW_EVENT);
        } else {
            if (!unit.isBound()) {
                unit.set(UNIT);
            }
        }
    }
Sets the unit of the tile. This unit will usually only be
visible if it is not empty.

@param UNIT
src/main/java/eu/hansolo/tilesfx/Tile.java