How to iterate through JSONArray which has JSONArray and JSONObject inside Java 8

Question

Could anyone tell me how to iterate through JSONArray which returns both JSONArray and JSONObject in it. I tried below code and I'm getting error as below.

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of 'com.example.jsonarr.pojoClass[]' out of START_OBJECT token

Code

List<pojoClass> pojoClassList = new ArrayList();
JSONArray jsonArrayList = new JSONArray( jsonResponse );
ObjectMapper objectMapper = new ObjectMapper(  );
pojoClassList = (List)objectMapper.readValue(jsonArrayList.toString(),
                        objectMapper.getTypeFactory().constructCollectionType(List.class, pojoClass[].class));

JSONArray

[
  {
  "Key1": "Value1",
  "Key2": "Value2",
  "Key3": "Value3",
  "Value1_tim":       {
     "amVal": 0,
     "pmVal": "0"
    }
  },
  [   {
  "Key1": "Value1",
  "Key2": "Value2",
  "Key3": "Value3",
  "Value1_tim":       {
     "amVal": 0,
     "pmVal": "0"
  }
  }]
]

With normal for loop.

for ( int i = 0; i < jsonArrayList.length(); i++ ) {
     JSONObject jsonObject = jsonArrayList.optJSONObject( i );
     if ( jsonObject != null ) {
        pojoClass = objectMapper.readValue( jsonObject.toString(), PojoClass.class );
           }
     if ( jsonObject == null ) {
        JSONArray jsonArrayInner = new JSONArray( jsonArrayList.getJSONArray( i ).toString() );
        for ( int j = 0; j < jsonArrayInner.length(); j++ ) {
         JSONObject jsonObject1 = jsonArrayList.optJSONObject( j );
           if ( jsonObject1 != null ) {
            pojoClass = objectMapper.readValue( jsonObject1.toString(), PojoClass.class );
                 }
             }
         }
    pojoClassList.add( pojoClass );
  }

How do I do that with Java 8?

You can check this link stackoverflow.com/questions/33215539/… — Venky1729
– Venky1729, Commented Apr 10, 2019 at 7:50

Michał Ziober · Accepted Answer · 2019-04-10 10:44:37Z

0

If you use Jackson's ObjectMapper try to use ACCEPT_SINGLE_VALUE_AS_ARRAY feature which allows to treat single elements as one-element-array. Below, you can find simple example how to read your JSON to list of Pojo classes:

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.type.CollectionType;

import java.io.File;
import java.util.List;
import java.util.stream.Collectors;

public class JsonApp {

    public static void main(String[] args) throws Exception {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();

        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);
        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);
        mapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);

        CollectionType collectionType0 = mapper.getTypeFactory().constructCollectionType(List.class, Pojo.class);
        CollectionType collectionType1 = mapper.getTypeFactory().constructCollectionType(List.class, collectionType0);
        List<List<Pojo>> list = mapper.readValue(jsonFile, collectionType1);

        List<Pojo> pojos = list.stream()
                .flatMap(List::stream)
                .collect(Collectors.toList());
        System.out.println(pojos);
    }
}

class Pojo {

    @JsonProperty("Key1")
    private String key1;

    // getters, setters, toString
}

Above code prints:

[Pojo{key1='Value1'}, Pojo{key1='Value1-1'}]

answered Apr 10, 2019 at 10:44

Michał Ziober

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7 Comments

Sam Over a year ago

I'm still getting an error com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of java.lang.String` out of START_ARRAY token`

Sam Over a year ago

It's failing in line List<List<Pojo>> list = mapper.readValue(jsonFile, collectionType1);

Michał Ziober Over a year ago

@Sam, I have tested it for JSON payload you included to a question. Have you tested it for the same JSON?

Sam Over a year ago

Yes, its the same payload. It's of type JSONArray. It's working fine when it returns JSONObject, however the second value is of JSONArray.

Michał Ziober Over a year ago

@Sam, which version of Jackson do you use? I've tested it for the latest one.

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