What structure to choose for an array where the order is not important, but is determined only by the elements?

Question

Java

For example, simulating a road:

class Road {
City city1, city2;
}

But then Road(Spb, Msk) and Road(Msk. Spb) modeling the same track will be two different objects. How can it be better then?

Answer 1

There are several options for storing undirected graphs.
1. Store any of the roads, and when accessing them, check for the presence of one of the 2
2. Always store only the road from the city with a lower number to the city with a larger one.
3. Keep 2 ways always.
Depending on the task, it may be more convenient to use one of these methods.

Answer 2

You do not need to create a new object, but create a selector method that will check the existence of the road and if it exists, return the existing one, if not, then create a new one. In this case, you can implement the check like this:

Set<Road> roads = new HashSet<Road>();
Road getRoad(c1, c2){
for(Road r:roads) if((r.city1.equals(c1) && r.city2.equals(c2)) || (r.city2.equals(c1) && r.city1.equals(c2))) return r;
Road r = new Road(c1, c2);
roads.add(r);
return r;
}

If there are more cities, it makes sense to use sets instead of listing cities in separate variables.
Ps I wrote from a mobile phone, there may be errors

Answer 3

Yes, even if the road passes through 10 cities - use TreeSet().

SortedSet road1 = TreeSet<City>();
road1.add("Mos");
road1.add("Spb");

SortedSet road2 = TreeSet<City>();
road2.add("Spb");
road2.add("Mos");

assert road1.equals(road2);