@Override
    public void draw(final BoundingBox boundingBox, final byte zoomLevel, final Canvas canvas, final Point topLeftPoint, final Rotation rotation) {
        layerLock.lock();
        try {
            for (Map<Integer, Layer> zLevelMap : this.layerItemMap.values()) {
                for (Layer entry : zLevelMap.values()) {
                    if (entry != null) {
                        entry.draw(boundingBox, zoomLevel, canvas, topLeftPoint, rotation);
                    }
                }
            }
        } finally {
            layerLock.unlock();
        }
    }
adds layers to this group with a given zLevel
main/src/main/java/cgeo/geocaching/unifiedmap/geoitemlayer/MapsforgeV6ZLevelGroupLayer.java