Question

I have a 2D array of Integers. I want them to be put into a HashMap. But I want to access the elements from the HashMap based on Array Index. Something like:

For A[2][5], map.get(2,5) which returns a value associated with that key. But how do I create a hashMap with a pair of keys? Or in general, multiple keys: Map<((key1, key2,..,keyN), Value) in a way that I can access the element with using get(key1,key2,…keyN).

EDIT : 3 years after posting the question, I want to add a bit more to it

I came across another way for NxN matrix.

Array indices, i and j can be represented as a single key the following way:

int key = i * N + j;
//map.put(key, a[i][j]); // queue.add(key); 

And the indices can be retrevied from the key in this way:

int i = key / N;
int j = key % N;

Answer 1

There are several options:

2 dimensions

Map of maps

Map<Integer, Map<Integer, V>> map = //...
//...

map.get(2).get(5);

Wrapper key object

public class Key {

    private final int x;
    private final int y;

    public Key(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Key)) return false;
        Key key = (Key) o;
        return x == key.x && y == key.y;
    }

    @Override
    public int hashCode() {
        int result = x;
        result = 31 * result + y;
        return result;
    }

}

Implementing equals() and hashCode() is crucial here. Then you simply use:

Map<Key, V> map = //...

and:

map.get(new Key(2, 5));

Table from Guava

Table<Integer, Integer, V> table = HashBasedTable.create();
//...

table.get(2, 5);

Table uses map of maps underneath.

N dimensions

Notice that special Key class is the only approach that scales to n-dimensions. You might also consider:

Map<List<Integer>, V> map = //...

but that’s terrible from performance perspective, as well as readability and correctness (no easy way to enforce list size).

Maybe take a look at Scala where you have tuples and case classes (replacing whole Key class with one-liner).