Question

I want to store an Integer or a Double, both extending from Number, into a HashMap, and retrieve them without casting. The reason is that I’ll eventually have many have many different subtypes (I am only using Number/Integer for illustration), and I don’t want to have to write a new method when a different subtype is introduced. Can I structure and call getFromMap() below without having to do the cast to Double or Integer ?

public class MapOfContent {
    static Map<String, Number> hm = new HashMap<String, Number>();

    public static void main(String[] args) {
        addToMap(hm, Integer.valueOf(3), "An Integer");
        addToMap(hm, Double.valueOf(3.4), "A Double");

        Double d;

        d = (Double) getFromMap(hm, "A Double");
        //d = getFromMap(hm, "A Double");

        System.out.println("Result-> " + d);

    }

    private static void addToMap(Map<String, ? super Number> hashmap, Number c, String key) {
        hashmap.put(key, c);
    }

    private static <T> Number getFromMap(Map<String, ? extends Number> hashmap, String cmsKey) {
        return hashmap.get(cmsKey);
    }
}

Answer 1

There’s no way to actually make this approach genuinely typesafe; what if you tried (Double) getFromMap(hm, "An Integer")? That should fail.

One potential alternative might be to use the actual Class object as a key, for example using a Guava ClassToInstanceMap:

ClassToInstanceMap<Number> map = MutableClassToInstanceMap.create();
map.putInstance(Integer.class, Integer.valueOf(3));
map.putInstance(Double.class, Double.valueOf(3.4));

Double aDouble = map.getInstance(Double.class); // no cast required