将map.values（）返回的Collection转换为List

Question

I have tried everything I knew and whatever I found online but it just is not working. I keep getting:

java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.mycompany.myproject.dto.MyDto

I would appreciate if somebody could please tell me what I am doing wrong in the code below:

List<Map<Long, MyDto>> dtoList = restTemplate.getForObject(myRestUrl + "/some-path/dtoInfo/" + ids, List.class);
Map<Long, MyDto> myMap = dtoList.get(0);
System.out.println("myMap SIZE is: " + myMap.size());
System.out.println("myMap is: " + myMap);

List<MyDto> dtos = new ArrayList<MyDto>(myMap.values());
StringJoiner sj = new StringJoiner(",");
for(MyDto obj : dtos) {
    sjStoreIds.add(obj.getId());
}

The REST call comes back with a List containing a HashMap object with the "Key" of type Long and the "Value" of type MyDto . Then I am printing the "Size" of the map and the map itself. Both prints the correct and expected information.

Then I am extracting the list of values from the map as below:

List<MyDto> dtos = new ArrayList<MyDto>(myMap.values());

No issues there.

However, when I am trying to go through the object in the converted list (dtos) as below:

for(MyDto obj : dtos) {
    sjStoreIds.add(obj.getId());
}

I get java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.mycompany.myproject.dto.MyDto for for(MyDto obj : dtos) { part.

Spinning my brains for more than 2 hours now. :(

I am sure it's going to something very basic and I will be pissed and embarrassed once I know it but can anybody please tell me where I am doing wrong?

EDIT

Following is the REST implementation I have server-side:

    @Override
    public List<Map<Long, MyDto>> getInfo(String idValues) {
        List<Map<Long, MyDto>> returnList = new ArrayList<>();
        Map<Long, MyDto> result = new HashMap<>();

        String[] ids = idValues.split(",");
        for(String id : ids) {
            result.put(Long.parseLong(id), testInfo.get(Long.parseLong(id)));
        }

        returnList.add(result);

        return returnList;
    }

The " testInfo " above is a map that gets built as result of SQL query.

Answer 1

restTemplate.getForObject is returning you something that isn't actually a List<Map<Long, MyDto>> . All the code you've shown is typesafe and correct, but getForObject silently skips type checking in its signature, and that's biting you. Figure out what type it really is. (From what you've shown us, it's probably a List<Map<Long, Map<Something, Something>>> .)