Java generics enforce same type for keys and values of map

Question

What I am looking for:

I'm looking to construct something that enforces type on both the keys and values of a map: kind of like Map<Key<X>, Value<X>>. However, I would additionally like to enforce that types match within each key/value entry, but between entries, no type must should be enforced.

For example, within the same map, these key/value pairs should be considered valid:

• Key<Integer> maps to Value<Integer>
• Key<String> maps to Value<String>
• Key<Double> maps to Value<Double>

However, something like this would be invalid:

• Key<Integer> mapping to Value<String>
• Key<Double> mapping to Value<Boolean>

How can I accomplish this using Java generics?

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What I'm not looking for:

• I understand that I can implement something like Set<Pair>, where pair accepts Key/Value of the same type. However, looking this up by key would no longer be a constant time operation.

• I understand that I could do something like Map<Key<?>, Value<?>> and just assert that the Key and Value are the same type at runtime. However, I was wondering if this is possible strictly using generics.

Answer 1

You can do this, but you have to roll your own wrapper on top of a Map:

class MyTypeSafeMap {   private Map<Key<?>, Value<?>> map;   public <T> void put(Key<T> key, Value<T> value) {     map.put(key, value);    }    public <T> Value<T> get(Key<T> key) {     return (Value) map.get(key);      // we know it's safe, but the compiler can't prove it   } } 

Compare e.g. Guava's ClassToInstanceMap.