Question

Is it possible to create a HashMap whose key is an array of integers?

I´m accustomed to use Python and just recently I began working with Java. In my job I need to create a HashMap with keys like:

map.put([5,2], 1);
map.put([3,2], 0);

and so on. I´m using it to later on test if a pair of those numbers are present in the map, if yes, then do something, if not, continue.

To do so I tried the following:

Map<Array, Boolean> test = new HashMap<Array, Boolean>();
int[] a1 = {5,2};
test.put(a1, true);

Eclipse gives the message (“The arguments are not applicable to int[]…”). But any configuration I´ve done I get some error.

I tried using ArrayList, Objects inside the map, nested HashMap and so on but none worked (in python it´s very easy, I´d just write dict[(5,2)] = 1, so I imagine in Java there´s something simple like that). I was suggested to transform the numbers into Strings and add a colon between then, like:

map.put("5:2", 1);

and later on I break the string again but if this is the solution I´ll go back to Python ;)!!

Probably this is a very simple question but I couldn´t find the answer, hope you can help me.

Thanks in advance!

Answer 1

If you want to check for the existance of your entry, you can use a Set (a useful concrete implementation is HashSet.

final Set<List<Integer>> population;

population = new HashSet<List<Integer>>();
population.add(Arrays.asList(1, 2));

if (population.contains(Arrays.asList(1, 2)))
{
  ...
}

You can use an List as I have done above – but that doesn’t guarantee that all your lists are exactly two elements long (if that is indeed a constraint). To make it a bit more robust, you could create your own class to represent the tuple. If you do, make sure you implement equals() and hashCode() (here’s an article explaining good practice).

Arrays.asList() is a useful way of creating a list in-line in the code. A more general list is an ArrayList.