Enforce specific Map implementation in Immutables

Question

I am using Immutables Java library, how can I enforce a specific map implementation, without having to use a specific reference?

@Immutable
public interface ConfigIF {
  Map<String, String> getOptions();
}

If I use the above code the concrete Map is a LinkedHashMap.

@Immutable
public interface ConfigIF {
  TreeMap<String, String> getOptions();
}

If I use the above code both the reference and the implementations are TreeMap.

Is there a way to specify Map as a reference but enforce TreeMap as concrete type?

Answer 1

Absolutely not - and it should be forbidden (that is : no option given to you as a user to alter that). This is not specific to your library, but any code in general that returns a Map instead of TreeMap let' say.

Look it the other way, if you return a Map users are not expect to call ceilingEntry on it (that is available in TreeMap, but not in Map) via cast of the reference.

java-8 does the same thing when via it's Collectors.toXXX. For example Collectors.toList explicitly says:

There are no guarantees on the type, mutability, serializability, or thread-safety of the List returned.

This means that they are allowed to return whatever they find the most appropriate.