java.lang.String out of START_ARRAY Error while converting POJO to HasMap<String,String> using ObjectMapper

Question

@Builder
@Getter
public class POJOClass {
    @NonNull
    private List<String> states;

    @NonNull
    private String country;

    @NonNull
    private String capital;

    public Map<String,String> convertToMap() {
        TypeReference<HashMap<String, String>> hashMapType
                = new TypeReference<HashMap<String, String>>() {};
        return new ObjectMapper().convertValue(this, hashMapType);
    }
}

i am trying to convert an instance of the above class into a HashMap<String,String> using instance.convertToMap(), but this fails with error

Can not deserialize instance of java.lang.String out of START_ARRAY token
    at [Source: N/A; line: -1, column: -1] (through reference chain: java.util.HashMap["states"])
    at com.fasterxml.jackson.databind.ObjectMapper._convert(ObjectMapper.java:3605)
    at com.fasterxml.jackson.databind.ObjectMapper.convertValue(ObjectMapper.java:3546)
    at com.fasterxml.jackson.databind.ObjectMapper.convertValue(ObjectMapper.java:3534)

I believe this is because of the List when deserialising to map isnt getting converted to String.

Is there a way where i can convert a POJO to Map<String,String> and i dont want a type of Map<String, Object> ?

Can you elaborate a little more ? I am trying to convert an instance of the class to the Map<String,String> without me dealing directly with the json of it. — achut1993
– achut1993, Commented Dec 27, 2017 at 20:00
what output are you expecting? a List doesn't naturally convert into a Map irrespective of the mechanism — AdamPillingTech
– AdamPillingTech, Commented Dec 27, 2017 at 20:37
I need the output of the method convertToMap be a HasMap<String,String> . I need the list to be converted into a string and be stored in the HasMap<String,string> — achut1993
– achut1993, Commented Dec 28, 2017 at 4:44

Andrei Sfat · Accepted Answer · 2017-12-28 16:47:57Z

1

You cannot do the TypeReference of type Map<String, String> since states is not a string, but a list, so you can change the TypeReference into Map<String,Object> and this should over even if you have a list or if you want in the future to add a complex object

public Map<String, String> convertToMap() {
    TypeReference<HashMap<String, Object>> hashMapType = new TypeReference<HashMap<String, Object>>() {
    };
    return new ObjectMapper().convertValue(this, hashMapType);
}

UPDATE:

You can still leave the Map<String, String> as the return type of the method convertToMap()

Full example:

@Builder
@Getter
public class POJOClass {
    @NonNull
    private List<String> states;

    @NonNull
    private String country;

    @NonNull
    private String capital;

    public Map<String, String> convertToMap() {
        TypeReference<HashMap<String, Object>> hashMapType = new TypeReference<HashMap<String, Object>>() {
        };
        return new ObjectMapper().convertValue(this, hashMapType);
    }


    public static void main(String[] args) {
        POJOClass pojoClass = new POJOClass(Lists.newArrayList("a", "b", "c"), "England", "London");
        System.out.println(pojoClass.convertToMap());
    }
}

edited Dec 28, 2017 at 16:47

answered Dec 27, 2017 at 21:15

Andrei Sfat

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8 Comments

achut1993 Over a year ago

if I have the type reference as map<string,object> . then wouldn't the map returned by the convertToMap method also be a HashMap<String,Object> ?

achut1993 Over a year ago

but there is a constraint that i need to return a HashMap<String, String> irrespective of the type of fields the POJOClass has. So, is there any way i can return a HashMap<String, String> class, like some how can i convert the list to string while creating the HashMap ?

Andrei Sfat Over a year ago

@achut1993, as you can see, I am returning Map<String, String>. I am just doing the convert to Map<String, Object> only when doing the TypeReference. This works fine. I will update my answer to show you an example

achut1993 Over a year ago

This doesnt work as if you do pojoClass.convertToMap().get("states") it is a List not String.

Andrei Sfat Over a year ago

@achut1993 List<String> states = Collections.singletonList(pojoClass.convertToMap().get("states"));

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