Why does Gson Preconditions fail in this instance?

Question

I am writing a deserializer for the class ForeignCollection, which is abstract, and provided to me in the ORMLite Android Library. See my code below.

public class ForeignCollectionDeserializer implements JsonDeserializer<ForeignCollectionDeserializer> {
    @Override
    public ForeignCollection deserialize(JsonElement jsonElement, Type type, JsonDeserializationContext jsonDeserializationContext) throws JsonParseException {
        ...
    }
}

I have also tried implementing JsonDeserializer<?> and JsonDeserialzer (with no generic type)

I'm using GsonBuilder.registerTypeAdapter to register the deserializer.

GsonBuilder gb = new GsonBuilder();

gb.registerTypeAdapter(ForeignCollection.class, new ForeignCollectionDeserializer());
Gson gson = gb.create();

However, registerTypeAdapter() is throwing this:

06-11 11:27:27.937: E/AndroidRuntime(15197): FATAL EXCEPTION: main
06-11 11:27:27.937: E/AndroidRuntime(15197): Process: com.myapp, PID: 15197
06-11 11:27:27.937: E/AndroidRuntime(15197): java.lang.IllegalArgumentException
06-11 11:27:27.937: E/AndroidRuntime(15197):    at com.google.gson.internal.$Gson$Preconditions.checkArgument($Gson$Preconditions.java:42)
06-11 11:27:27.937: E/AndroidRuntime(15197):    at com.google.gson.GsonBuilder.registerTypeAdapter(GsonBuilder.java:448)

I am stepping through the code and I get to the offending piece, from GsonBuilder below.

public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
    $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
            || typeAdapter instanceof JsonDeserializer<?>
            || typeAdapter instanceof InstanceCreator<?>
            || typeAdapter instanceof TypeAdapter<?>);
    if (typeAdapter instanceof InstanceCreator<?>) {
        instanceCreators.put(type, (InstanceCreator) typeAdapter);
    }

I evaluated typeAdapter instanceof JsonDeserializer<?> in debug mode and found that it is actually false

Actually I've evaluated all of these:

• typeAdapter instanceof JsonDeserializer
• typeAdapter instanceof JsonDeserializer<ForeignCollection>
• typeAdapter instanceof JsonDeserializer<?>

They are all false.

How is this so, when I have declared typeAdapter as an Object of type ForeignCollectionsDeserializer, which implements JsonDeserializer?

Answer 1

It turns out that I had multiple incorrect imports.

I was accidentally importing another libraries implementation of Gson's JsonDeserializer, which caused the code to compile, but fail the instanceof conditional.

smh