Convert a List of JSON-objects input into a nested Map

Question

I have a String input in the following format:

Input String: [{ "id":"1", "name":"A", "address":"St 1"},{ "id":"2", "name":"B", "address":"St 2"}, ...]

And I want to be able to convert this to a Map<String, Map<String, String>> format. So, something like:

Required Output Format: {1: {id:1, name:"A", address: "St 1"} 2: {id:2, name:"B", address: "St 2"}}

I created a class to help in parsing the input:

public class Student{
  private String id;
  private String name;
  private String address;
}

Which I am trying to do through Jackson's ObjectMapper to get the data in the format: List<Student> format and then use Collectors.toMap() to convert it to a Map of Maps format.

All the examples I have seen so far suggest an approach like:

List<Student> student = objectMapper.readValue(inputString, new TypeReference<List<Student>>(){});

Map<String, Student> studentTempMap = student.stream()
    .collect(Collectors.toMap(
        Student::getId,
        Function.identity()
    ));

Which makes the studentTempMap something like:

{ 1: object of Student("id":"1", "name":"A", "address":"St 1"),
  2: object of Student("id":"2", "name":"B", "address":"St 2"), 
  ... }

A brute force approach from here:

create a new Map studentMap
Iterate over keys (which are "id") in studentTempMap.
- Then create another Map, temp.
- Add keys "id", "name", and "address" and values using something like studentTempMap.get(2).get("id"), and something similar for all the other keys (name and address). Where 2 would be the current iterator key over the Map studentTempMap.
- Finally add a key say as 2 (current iterator) and value temp in the studentMap.

I do not want to use this brute force approach as I have a large number of Student objects.

Is there a way through ObjectMapper to get the output directly in the form of Map<String, Map<String, String>> format? Or is there a way through Collectors.toMap to parse it to the above format?

Alexander Ivanchenko · Accepted Answer · 2022-09-18 02:01:52Z

2

I want to be able to convert this to a Map<String, Map<String, String>>

If you want to obtain a nested map of strings Map<String,Map<String,String>> as a result, you don't need to convert JSON into a list of POJO.

Instead, you can parse JSON into a list of maps List<Map<String,String>> and then generate a nested map.

String inputString = """
    [{ "id":"1", "name":"A", "address":"St 1"},
     { "id":"2", "name":"B", "address":"St 2"}]""";
    
ObjectMapper objectMapper = new ObjectMapper();
List<Map<String, String>> students = objectMapper.readValue(
    inputString, new TypeReference<>() {}
);
        
Map<String, Map<String, String>> studentMapById = students.stream()
    .collect(Collectors.toMap(
        map -> map.get("id"), // key 
        Function.identity(),  // value
        (left, right) -> left // resolving duplicates
    ));
    
studentMapById.forEach((k, v) -> System.out.println(k + " : " + v));

Output:

1 : {id=1, name=A, address=St 1}
2 : {id=2, name=B, address=St 2}

edited Sep 18, 2022 at 2:01

answered Sep 18, 2022 at 1:30

Alexander Ivanchenko

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4 Comments

iamgirdhar Over a year ago

Great solution. But there is one compile time error in new TypeReference<> as cannot use '<>' with anonymous inner classes.

Alexander Ivanchenko Over a year ago

@girdhar There's no compilation error - I've tested this solution. And there's no issue with using diamond operator and anonymous classes.

iamgirdhar Over a year ago

Yes you are right, actually I am using JDK 1.8 and that's why I am getting this error. This change for anonymous classes has been done in JDK 1.9 onwards. "Java 9 – Diamond operator enhancements"

Alexander Ivanchenko Over a year ago

@girdhar "I am using JDK 1.8" - you've missed a couple of LTS versions. Anyway, even with JDK 8 adding a type variable isn't a big deal.

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