[Solved] HashMap of ArrayList as the value initialization

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The List<Pack> in your HashMaps are never defined. They are of the interface list but nowhere you identify what kind of List they are. You have to follow a structure as follows:

Map<long, List<Pack>> map = new HashMap<>() // java 7 syntax
List<Pack> packets = new ArrayList<>();
packets.add(pack);
map.put(msg.getAck(), packets);

When adding an element to the ´List´, which you now can because it exists, you do:

map.get(msg.getAck()).add(pack);

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solved HashMap of ArrayList as the value initialization