How to determine Container template classes

Question

Working in Java 1.8, I need to determine the class of K and V in HashMap. My first attempt has not worked, and I admit I have no idea where to go from here. I am hoping to feed this

public static int foo(Object obj){
    int result = 0;
    String x = obj.getClass().getSimpleName();

    switch(x){
        case "HashMap":
            Type sooper = obj.getClass().getGenericSuperclass();
            Type key = ((ParameterizedType)sooper).getActualTypeArguments()[ 0 ];
            Type value = ((ParameterizedType)sooper).getActualTypeArguments()[ 1 ];
            System.out.println(sooper.getTypeName());
            System.out.println(key.getTypeName());
            System.out.println(value.getTypeName());

            break;
      }
}

public static void main(String[] args) {
    foo(new HashMap<Integer,Character>());
}

Output:

Class ::    HashMap
java.util.AbstractMap<K, V>
K
V

Have you look at: https://stackoverflow.com/questions/3403909/get-generic-type-of-class-at-runtime ? — Jorn Vernee, Jun 01 '17 at 12:10
Thanks for link. Unless I am mistaken, this works when you know the declared class. I can't do something like GenericClass(new HashMap). That would tell me it is a HashMap, and I am back to the original problem. — BAMF4bacon, Jun 01 '17 at 12:27
One other option is to get some elements from the map and call `getClass` on those, but there is no guarantee that those are of the exact types `K` and `V` (could be a subtype). Otherwise it's not possible. — Jorn Vernee, Jun 01 '17 at 12:30

score 0 · Answer 1 · answered Jun 01 '17 at 12:14

0

Typically you will not be able to retrieve the types of K and V due to type erasure. After you've created the template object, the system doesn't see it as <unique_type K, unique_type V> but rather <Object K, Object V>

I did some looking and found another question here on SO that offers an ugly, but working solution to the problem.

answered Jun 01 '17 at 12:14

Llebac

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2

I've tried several solutions there, and none of them worked. I'm going with "Impossible" Thanks everyone. – BAMF4bacon Jun 01 '17 at 12:36

score 0 · Answer 2 · answered Jun 01 '17 at 14:57

0

I solved this on my own by using an Enum type and passing this as an argument to my function, because the function caller knew the class in this case.

answered Jun 01 '17 at 14:57

BAMF4bacon

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How to determine Container template classes

2 Answers2