如何将 java.util.LinkedHashMap<*, *> 分配给 kotlin.collections.Map<*, *>

Question

I have declared a method which takes map as parameter:

fun myKotlinMethod(myParameter:Map<String,String>)...

Later I read data from json. I get an object of type: java.util.LinkedHashMap<*, *> I need to use reflection to invoke myKotlinMethod . Before that, I test if the value is assignable to myParameter . I ask parameter type using kotlin reflection and I get kotlin.collections.Map<*, *> .

bottom line is that java.util.LinkedHashMap<*, *> does not implement kotlin.collections.Map<*, *> (it cannot) and therefore is not assignable.

I'm using this method to check assignability

inline fun isAssignable(parameterType: KType, valueType: KType): Boolean {
    if (parameterType == valueType) return true
    return parameterType.isSubtypeOf(valueType)
}

and I test like

if (isAssignable(parameter.type.classifier!!.starProjectedType, parameterValue::class.starProjectedType))

I read about some dark magic kotlin does to collection during compile time, but during runtime, java classes should be used. This is not my case since I get kotlin classes in runtime.

So, How do I correctly check for collection types compatibility?

Answer 1

You seem to have written this the other way around:

// you can even delete the parameterType == valueType check
// isSubtypeOf returns true if the types are the same
return parameterType.isSubtypeOf(valueType)

The method call is valid if the type of the value is a subtype of the parameter type, so it should be:

return valueType.isSubtypeOf(parameterType)

However, note that this is not safe at all. Since you only know that the value is a LinkedHashMap<*, *> , what if the map has Int s, Double s, or UUID s in it? That would still pass the subtype check. Because you used starProjectedType on the parameter type, you threw away the <String, String> part of the type, and didn't actually check that the value actually only has strings.

To make sure that the collections are compatible, you should also check the collection's elements:

fun isAssignableToMapStringString(value: Any) =
    if (value is Map<*, *>) {
        value.all { it.value is String && it.key is String }
    } else {
        false
    }

It can be quite hard to generalise this to any KType though. You'd have to do a lot of reflection to get the elements out of the collection. And you'd have to handle maps and collections separately, since Map and Collection are unrelated interfaces :(