Adjacency Matrix Implementation

Question

I'm looking for an implementation of a Matrix with keys at first row and first column. First I thought something like a HashMap in a HashMap but this looks ugly and I think it would be wrong. I really need those keys where I can check, if a string exists in the first row as a key. (I want to implement an adjacency matrix)

I'm not sure if I was clear enough. Here a small Picture to visualize it.

Something like this:

Just that the first column and row are not numbers but cities.

In fact I want to save the distances from cities to cities in a adjacency matrix.

Answer 1

Use a custom class as a key in HashMap.

public class Key {
    private String city1;
    private String city2;

    public Key(String city1, String city2){
        this.city1 = city1;
        this.city2 = city2;
    }

    //hashCode and equals
}

then you can use it as so:

HashMap<Key, Integer> adjacencyMatrix = new HashMap<>();
adjacencyMatrix.put(new Key("Berlin", "London"), 933);

Integer distance = adjacencyMatrix.get(new Key("Berlin", "Paris"));
if (distance == null){
    //no entry
}

Don't forget to generate hashCode and equals in Key class!