Kotlin: objectmapper.readValue() with TypeReference<HashMap<String, String>> cannot infer parameter

I would like to deserialize a json to Map<String, String> with objectmapper with the following code:


fun onMessage(topic: String, message: MqttMessage) {
val typeRef = object : TypeReference<HashMap<String, String>>() {}
val msg = objectMapper.readValue(message.payload, typeRef)
...
}

Compiler says it connot infer parameter T in `fun <T : Any!> readValue (src: ByteArray!, valueTypeRef: (TypeReference<Any!>..TypeReference<*>?)): T!`

Is there any solution to this problem whitout extending a HashMap with my custom class like this:


class MyHashMap : HashMap<String, String>()


...


fun onMessage(topic: String, message: MqttMessage) {
val msg = objectMapper.readValue(message.payload, MyHashMap::class.java)
...
}

One possible way:

inline fun <reified T> ObjectMapper.readValue(s: String): T = this.readValue(s, object : TypeReference<T>() {})

val msg: Map<String,String> = objectMapper.readValue(message.payload)

The issue, really, is in Jackson's API here. Here's how the `readValue` method is declared:

public <T> T readValue(String content, TypeReference valueTypeRef)

They are using the raw type of `TypeReference` for some reason, even though they could easily take a `TypeReference<T>` as their parameter. If they did, you code would work as is, as Kotlin could then infer the `T` generic type parameter of the function, and therefore know its return type.

You can work around this issue a couple different ways, however, by being explicit with your types.

- Either by providing the generic type parameter for the function, and having the type of the `msg` variable inferred:

val typeRef: TypeReference<Map<K, V>> = object : TypeReference<Map<K, V>>() {}
val msg = objectMapper.readValue<HashMap<String, String>>(message.payload, typeRef)

- Or alternatively, by explicitly typing your variable, and having the function's type parameter inferred:

val msg: HashMap<String, String> = objectMapper.readValue(message.payload, typeRef)

You can use an [object expression](

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) to pass an anonymous implementation of `TypeReference`:

objectMapper.readValue(message.payload, object : TypeReference<HashMap<String, String>>() {})

Notice the `object` keyword before the `TypeReference` parameter.