Add EnumMap.keyType() and EnumSet.elementType()

Remi Forax forax at
Sat Dec 9 12:32:47 UTC 2017

Hi Andrej,
I do not think it's a good idea.

It's like exposing reified type arguments, i can see these kind of change back firing in at least two ways.
It's not future proof, part of valhalla requires to change generics to allow generics of value types which requires a form of reification, but we may end up with a light form of reification similar to Scala type manifest (type arguments are available at construction time but not after that point) exposing type arguments for EnumSet will be seen as a mismatch in that case. Also, we may want to introduce unmodifiable EnumSet in the future, currently an unmodifiable empty EnumSet do not need to store any type argument, the proposed API force us to have as many empty EnumSet as type arguments combination (it's why you have one static field by type specialization in C#), again it makes the proposed API not future proof.


----- Mail original -----
> De: "Stephen Colebourne" <scolebourne at>
> À: "core-libs-dev" <core-libs-dev at>
> Envoyé: Jeudi 7 Décembre 2017 13:09:40
> Objet: Re: Add EnumMap.keyType() and EnumSet.elementType()

> I'm surprised I've never run into this. This seems like a simple and
> useful change.
> Stephen
> On 7 December 2017 at 11:40, Andrej Golovnin <andrej.golovnin at> wrote:
>> Hi all,
>> it would be nice if we would have access to the class of the enum type
>> used to create an EnumMap or an EnumSet.
>> This is usefull when you write a custom serialization library and
>> would like to serialize/deserialize an empty EnumMap or an empty
>> EnumSet. For the empty EnumSet there is a workaround to get the enum
>> class:
>> EnumSet<MyEnum> empty = EnumSet.noneOf(MyEnum.class);
>> EnumSet<MyEnum> tmp = EnumSet.complementOf(empty);
>> Class<?> elementType = tmp.iterator().next().getClass();
>> But this only works when the enum class has at least one enum. For
>> EnumMap there is no such workaround at all and we have to use the
>> Reflection API. And you know the warnings since Java 9 :-) :
>> WARNING: An illegal reflective access operation has occurred
>> WARNING: Illegal reflective access by c.r.r.k.EnumMapSerializer to
>> field java.util.EnumMap.keyType
>> WARNING: Please consider reporting this to the maintainers of
>> c.r.r.k.EnumMapSerializer
>> WARNING: Use --illegal-access=warn to enable warnings of further
>> illegal reflective access operations
>> WARNING: All illegal access operations will be denied in a future release
>> So to avoid this warning and to avoid that our application stops to
>> work with a future release of Java I would like to propose to add the
>> following two methods:
>> EnumMap:
>>     /**
>>      * Returns the {@code Class} object of the key type for this enum map.
>>      *
>>      * @return the {@code Class} object of the key type for this enum map.
>>      *
>>      * @since 10
>>      */
>>     public Class<K> keyType()
>> EnumSet:
>>     /**
>>      * Returns the {@code Class} object of all the elements of this set.
>>      *
>>      * @return the {@code Class} object of all the elements of this set.
>>      *
>>      * @since 10
>>      */
>>     public Class<E> elementType()
>> The suggested change is attached as diff.
>> Best reagrds,
> > Andrej Golovnin

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