cast List<Object> to List<Map<String, Object>>

Question

I should know this, but I can't understand this for some reason.

Why can I not cast a List of Objects List<Object> to a List of Maps List<Map<String, Object>>? Every object in the list is an object of type Map<String, Object>, so why is the casting not possible?

What I can do is create a new ArrayList<Map<String, Object>>(); and iterate over the list and add each item with a cast.

List<Object> dataList;
..
//Why doesn't this work?
List<Map<String, Object>> rxData = (List<Map<String, Object>>)dataList;

//This works, but is there a better way?
rxData = new ArrayList<Map<String, Object>>();
for (Object data : dataList) {
    rxData.add((Map<String, Object>)data);
}

Solution 1

You can just drop generic parameter by double-casting:

@SuppressWarnings("‌​unchecked")
List<Map<String, Object>> rxData = 
    (List<Map<String, Object>>) (List<?>) dataList;

What is going here is that you force compiler to not to check generic type by omitting it with first cast and then do unchecked cast to List<Map<String, Object>>. This is possible because java generics is non-refiable.

The original error is caused by fact that Object is not compatible with Map<> type and there is no such thing like covariant/contravariant types in java (unlike scala for example).

But there gonna be a problems if dataList contains not maps.

Solution 2

List<Object> and List<Map<String, Object>> are not compatible types, therefore you can't directly cast one to the other. You would have to first cast to to a common supertype and then downcast:

List<Map<String, Object>> rxData = (List<Map<String, Object>>)(List<?>)dataList;

Note that you'll still get a warning at compile time, because it's an unsafe cast.

Comments

[markspace]

Java generics FAQ: angelikalanger.com/GenericsFAQ/FAQSections/…?

[k5_]

No need for a raw type here, and the problem is not related to reifiablity.

[vsminkov]

@k5_ agree. thanks! but anyway that will be unchecked cast. I'll edit my answer

[Andy]

Thank you! OK, I think I understand, and I need to careful so that the list does not contain anything besides maps.

[ruakh]

You should remove the first sentence of your answer, or move it later in the answer: it's true, but not very relevant. (Non-reifiability is why a cast from List<?> to List<Map<String, Object>> is unchecked; but it has nothing to do with why you can't cast from List<Object> to List<Map<String, Object>>.)

[vsminkov]

@ruakh you are right. thank you! I'll edit my answer

[Lew Bloch]

Per Joshua Bloch, if you must use @SuppressWarnings("‌​unchecked"), then you must also comment the code there documenting how it is that you are certain that the cast is safe.

[ruakh]

@LewBloch: I agree; though note that this cast actually isn't safe in the way that Joshua Bloch means, so instead of a comment documenting why it's safe, it should have a comment documenting why it's being used despite not being safe.