private Map<Double, List<WaypointLink>> createWaypointLinksByZ() { Map<Double, List<WaypointLink>> waypointLinksByZMap = new TreeMap<Double, List<WaypointLink>>(); for (WaypointLink wayLink : waypointLinks) { Waypoint waypoint = waypointByWaypointIdMap.get(wayLink .getStartPointId()); if (waypointLinksByZMap.get(waypoint.getZ()) == null) { waypointLinksByZMap.put(waypoint.getZ(), new ArrayList<WaypointLink>()); } waypointLinksByZMap.get(waypoint.getZ()).add(wayLink); } return waypointLinksByZMap; }