How do I return a defined Map?

Question

I want to define a function which returns an instantiated map depending on the Key and Value class which I give it:

something like

Map<Integer, String> map = getMap(Integer.class, String.class);

but I am unsure how to define the getMap function in the correct way. I was thinking something like

<K,V> Map<K,V> getMap(Class<K> keyClass, Class<V> valClass) {
   Map<keyClass, valClass> retMap = new HashMap<keyClass, valClass);


   .... // add values to my map

   return retMap;
}

But this doesn't seem to work at all.

does anyone have any suggestions?

Answer 1

You are mixing the concepts. Map is a generic type. Generics in Java will be erased upon compilation. So you cannot set them in runtime as you did. The correct way could be like this:

static <K,V> Map<K,V> getMap() { return new HashMap<K, V>(); }

Then you can invoke it like:

Map<Integer, String> m = getMap();

Answer 2

The proper syntax is much simpler:

Map<K, V> retMap = new HashMap<K, V>();

Because K and V are the type placeholders. The Class<K> and Class<V> variables are not needed here; but you will need them so the compiler can infer the proper types on invocation.

Answer 3

You did it too complicated, but you are really close. This way you will sort it out:

    <K,V> Map<K,V> getMap() {
           Map<K, V> retMap = new HashMap<K, V>();
           // add entries into the map
           return retMap;
    }

Java has a pretty nice automatic type inference for generic methods.