What to do when properties of a HashMap and ArrrayList are neccessary?

Question

I have a data structure that stores various permission groups for a multiplayer game server. There is an instance of a "Rank" class for each permission group. Each rank also has a name. I need to be able to access all of the ranks both in a Iterable fashion and with a key.

There are three main ways I can think of doing this. I don't know which one is best as they all have pros and cons. The first method could have a significant performance impact as the getRank(String) method will be called quite often with anywhere from about 2-50 ranks. The second method could also be problematic as there are several instances, although not nearly as common, where it is useful to have such a method with high performance. The third method could be problematic if the list and map ever get out of sync, especially since it needs to be thread-safe.

I would like to know which method is best and why. My primary concern is that it is both reliable and performant.

Method #1

public synchronized Rank getRank(String name) throws IllegalArgumentException{
    for(Rank rank : ranks){
        if(rank.getName().equalsIgnoreCase(name)){
            return rank;
        }
    }

    throw new IllegalArgumentException("Rank: " + name + " does not exist!");
}

Method #2:

public synchronized List<Ranks> getRanks(){
    ArrayList<Ranks> ranksList = new ArrayList<Ranks();

    for(String rankName : ranksMap.keySet()){
        ranksList.add(ranksMap.get(rankName));
    }

    return Collections.unmodifiableList(ranksList); //unmodifiableList to prevent someone from trying to add or remove ranks by editing this list
}

Method #3:

public class RankList{
    private ArrayList<Rank> list = new ArrayList<>();
    private Map<String, Rank> map = new HashMap<>(); 

    public void add(Rank rank){
        list.add(rank);
        map.put(rank.getName().toLowerCase(), rank);
    }

    public List<Rank> getRanks(){
        return Collections.unmodifiableList(list);  
    }

    public Rank get(String name){
        return map.get(name.toLowerCase());
    }

}

Do you order by insertion order or something else? A list allows you to reorder elements, and the distinction is very important. — user22815
– user22815, Commented Oct 6, 2015 at 3:41

Community · Accepted Answer · 2017-05-23 12:40:17Z

If you order elements by insertion order, you can use Java's LinkedHashMap

You will not be able to view it as a list, but you can easily siphon data into a temporary list and return that list. From there you can sort it if necessary.

You can also use a SortedMap (TreeMap is its standard implementation) if you want to have the map in a specific order. You can return a list view the same way, copying data into a list via iterator.

Member variable declaration would be one of the following:

// Need insertion order
private Map<String, Rank> map = new LinkedHashMap<>();
// Need key order or some other order (can pass in comparator)
private Map<String, Rank> map = new TreeMap<>();
// Do not care about order.
private Map<String, Rank> map = new HashMap<>();

You would return the list one of these ways:

// Using map order or do not care about order.
public List<Rank> getRanks() {
  return new ArrayList<Rank>(map.values());
}

// Using some other order.
public List<Rank> getRanks() {
  List<Rank> values = new ArrayList<Rank>(map.values());
  Collections.sort(values); // can pass in comparator
  return values;
}

NB: I know you said order does not matter, but I am covering all the bases for other readers who might find this information useful.

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