I have some code that just takes in a Collection and checks for null if it not just converts it all toString for you.

static Collection<String> test1(Collection<Object> objects) {
        if(objects != null) {
            return objects.stream().map(x -> x.toString()).collect(Collectors.toList())
        }else {
            return null;
        }
    }

It works fine if I do this

Collection<Object> myCollectionOfObjects = new ArrayList<>();
test1(myCollectionOfObjects);

but java complains if I pass in a collection of Enums

Collection<SomeEnumIHave> myCollectionOfEnums = new ArrayList<>();
test1(myCollectionOfEnums);

The exception I get is

Error:(283, 87) java: incompatible types: java.util.Collection<SomeEnumIHave> cannot be converted to java.util.Collection<java.lang.Object>

But from reading online, it seems that Enums are actual reference types, so it should be an Object.

I have multiple ways of working around this, but I'm confused why it's happening.

Edit I should point out that any other object type works fine. So it's not an issue with generics (the method itself isn't generic anyway), it seems to be something like a boxing issue, but Enums shouldn't need to be boxed.

Collection<MyCustomObject> myCollectionOfObjects = new ArrayList<>();
test1(myCollectionOfObjects);