java - Casting LinkedHashMap to Complex Object

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I've got an application that stores some data in DynamoDB using Jackson to marshall my complex object into a JSON.
For example the object I'm marshalling might look like this:
private String aString;
private List<SomeObject> someObjectList;
Where SomeObject might look like this:
private int anInteger;
private SomeOtherObject;
and SomeOtherObject might look like this:
private long aLong;
private float aFloat; 
This is fine an the object gets marshalled no problem and stored in the DB as a JSON string.
When it comes time to retrieve the data from DynamoDB Jackson automatically retrieves the JSON and converts it back... EXCEPT that 'someObjectList' is returned as a List<LinkedHashMap> not as a List<SomeObject>! This is standard behaviour for Jackson, its not a mistake that this is happening.
So now this leads to a problem. My code base thinks its dealing with a List<SomeObject> but the reality is that its handling a List<LinkedHashMap>! My question is how do I get my LinkedHashMap back into a 'SomeObject'. Obviously this is a manual process but what I mean is I can't even extract the values.
If I do this:
for (LinkedHashMap lhm : someObjectList) {
    // Convert the values back
I get a compile error telling me that someObjectList is of type 'SomeObject' not LinkedHashMap.
If I do this:
for (SomeObject lhm : someObjectList) {
    // Convert the values back
I get a runtime error telling me that LinkedHashMap cannot be cast to 'SomeObject'.
You can use ObjectMapper.convertValue(), either value by value or even for the whole list. But you need to know the type to convert to:
POJO pojo = mapper.convertValue(singleObject, POJO.class);
// or:
List<POJO> pojos = mapper.convertValue(listOfObjects, new TypeReference<List<POJO>>() { });
this is functionally same as if you did:
byte[] json = mapper.writeValueAsBytes(singleObject);
POJO pojo = mapper.readValue(json, POJO.class);
but avoids actual serialization of data as JSON, instead using an in-memory event sequence as the intermediate step.
                Awesome. The only thing that I had to do is to make sure that the POJO class contain a default constructor (in case other parameterized constructors are present)
– Gnana
                Mar 3, 2016 at 22:33
                I mean that with the first example the type has to be known at design time. You cannot just pass the type of the object to the (de)serializing method, which is a problem if you are making one method to (de)serialize any kind of class. With the second example you can just use .getClass() for serialization and pass in the type for deserialization.
– Bo Søborg Petersen
                Apr 12, 2016 at 15:23
                @BoSøborgPetersen Ok, that is true with TypeReference,  but you can construct List type with TypeFactory (constructCollectionType(elementType) or such), not a general limitation.
– StaxMan
                Apr 14, 2016 at 4:08
                Just a quick note for new readers: The TypeReference class in the code snippet comes from "com.fasterxml.jackson.core.type.TypeReference"
– AliReza
                Jan 17, 2020 at 19:59
I had similar Issue where we have GenericResponse object containing list of values
 ResponseEntity<ResponseDTO> responseEntity = restTemplate.exchange(
                redisMatchedDriverUrl,
                HttpMethod.POST,
                requestEntity,
                ResponseDTO.class
Usage of objectMapper helped in converting LinkedHashMap into respective DTO objects
 ObjectMapper mapper = new ObjectMapper();
 List<DriverLocationDTO> driverlocationsList = mapper.convertValue(responseDTO.getData(), new TypeReference<List<DriverLocationDTO>>() { });
public static <T> List<T> getObjectList(final String json, final Class<T> cls) 
    return objectMapper
        .readValue(json, objectMapper.getTypeFactory()
        .constructCollectionType(ArrayList.class, cls));
                Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.
– Community
                Sep 7, 2022 at 6:43
                This solution really helped me generalize my API calls that have to return lists of objects of my class type.
– ychaulagain
                Dec 22, 2022 at 22:15
There is a good solution to this issue:
import com.fasterxml.jackson.databind.ObjectMapper;
ObjectMapper objectMapper = new ObjectMapper();
***DTO premierDriverInfoDTO = objectMapper.convertValue(jsonString, ***DTO.class); 
Map<String, String> map = objectMapper.convertValue(jsonString, Map.class);
Why did this issue occur? I guess you didn't specify the specific type when converting a string to the object, which is a class with a generic type, such as, User <T>.
Maybe there is another way to solve it, using Gson instead of  ObjectMapper. (or see here Deserializing Generic Types with GSON)
Gson gson = new GsonBuilder().create();
Type type = new TypeToken<BaseResponseDTO<List<PaymentSummaryDTO>>>(){}.getType();
BaseResponseDTO<List<PaymentSummaryDTO>> results = gson.fromJson(jsonString, type);
BigDecimal revenue = results.getResult().get(0).getRevenue();
                While this code snippet may be the solution, including an explanation really helps to improve the quality of your post. Remember that you are answering the question for readers in the future, and those people might not know the reasons for your code suggestion.
– Johan
                Mar 20, 2019 at 9:37
                @Johan OK, Thanks for the warning. I'm a novice for answering some question on Stack Overflow, so I'll do better in the future.
– Donald Choi
                Apr 19, 2019 at 7:12
I use com.fasterxml.jackson.databind.ObjectMapper to mapping from LinkedHashMap to Json string first and convert from json string to Object
Map<String, String> mappingValue
OBJECTA a = objectMapper.readValue(toJson(mappingValue), OBJECTA.class);
public static String toJson(Object object) {
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    String jsonString = "";
    try {
        jsonString = mapper.writeValueAsString(object);
    } catch (JsonProcessingException var4) {
        var4.printStackTrace();
        jsonString = "Can't build json from object";
    return jsonString;
                Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.
– Community
                Oct 2, 2022 at 12:22
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