Parse JSON into Object

Question

I have a User class defined as:

User.java

package model;

import java.util.List;
import java.util.Map;

public class User {
    private final Map<String, List<String>> accountTransactionsMap;

    public User(final Map<String, List<String>> accountTransactionsMap) {
        this.accountTransactionsMap = accountTransactionsMap;
    }

    public Map<String, List<String>> getAccountTransactionsMap() {
        return accountTransactionsMap;
    }
}

I am calling a REST API that returns the following response:

{  
   "username1":{  
      "456":[  

      ],
      "123":[  

      ],
      "789":[  

      ]
   },
   "username2":{  
      "123":[  

      ],
      "456":[  

      ],
      "789":[  

      ]
   },
   "username3":{  
      "789":[  

      ],
      "123":[  

      ],
      "456":[  
         "transaction10",
         "transaction6",
         "transaction9",
         "transaction3"
      ]
   }
}

I would like to be able to parse through the response and store it in a User object.

I have tried the following:

Test.java

public class Test {
    public static void main(final String[] args) {
        final String response = "{\"username1\":{\"456\":[],\"123\":[],\"789\":[]},\"username2\":{\"123\":[],\"456\":[],\"789\":[]},\"username3\":{\"789\":[],\"123\":[],\"456\":[\"transaction10\",\"transaction6\",\"transaction9\",\"transaction3\"]}}";
        final Gson gson = new Gson();
        final Type map = new TypeToken<Map<String, User>>(){}.getType();
        final Map<String, User> result = gson.fromJson(response, map);
        System.out.println(result);

        if (result != null) {
            for (final Map.Entry<String, User> entry : result.entrySet()) {
                System.out.println("username: " + entry.getKey());
                final User user = entry.getValue();
                System.out.println("transactions: " + user.getAccountTransactionsMap());
            }
        }
    }
}

This yields output:

{username1=model.User@80ec1f8, username2=model.User@1445d7f, username3=model.User@6a396c1e}
username: username1
transactions: null
username: username2
transactions: null
username: username3
transactions: null

I expect output:

{username1=model.User@80ec1f8, username2=model.User@1445d7f, username3=model.User@6a396c1e}
username: username1
transactions: {123=[],456=[],789=[]}
username: username2
transactions: {123=[],456=[],789=[]}
username: username3
transactions: {123=[],456=["transaction10", "transaction6", "transaction9", "transaction3"],789=[]}

How can I parse the accountId and the list of transactionIds into its own map as a variable in my User class?

Edit: I suppose the question really becomes, how can I create a custom TypeToken for my User class?

score 0 · Accepted Answer · answered Feb 22 '19 at 00:36

Instead of User class you need to use Map<String, Map<String, List<String>>>:

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.io.File;
import java.io.FileReader;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;

public class GsonApp {

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

        final Gson gson = new Gson();
        final Type map = new TypeToken<Map<String, Map<String, List<String>>>>(){}.getType();
        final Map<String, Map<String, List<String>>> result = gson.fromJson(new FileReader(jsonFile), map);
        System.out.println(result);

        if (result != null) {
            for (final Map.Entry<String, Map<String, List<String>>> entry : result.entrySet()) {
                System.out.println("username: " + entry.getKey());
                final Map<String, List<String>> user = entry.getValue();
                System.out.println("transactions: " + user);
            }
        }
    }
}

Above code prints:

{username1={456=[], 123=[], 789=[]}, username2={123=[], 456=[], 789=[]}, username3={789=[], 123=[], 456=[transaction10, transaction6, transaction9, transaction3]}}
username: username1
transactions: {456=[], 123=[], 789=[]}
username: username2
transactions: {123=[], 456=[], 789=[]}
username: username3
transactions: {789=[], 123=[], 456=[transaction10, transaction6, transaction9, transaction3]}

If you really need, you can create User object after parsing.

Thanks, this code is working, however if the JSON response changes then it will be difficult to maintain. I'm going to edit my post to ask if there's a way to define a custom `TypeToken` for a class so that I can instead pass `TypeToken` or something of the sort. — raphattack, Feb 22 '19 at 00:47
@raphattack, you can try to write custom deserialiser like it is pointed in this [question](https://stackoverflow.com/questions/10267749/how-to-parse-a-json-with-dynamic-key-in-android-by-using-gson). If you can use `Jackson` maybe try it. It has a little bit better deserialisation options. See this [question](https://stackoverflow.com/questions/6796662/using-gson-to-parse-a-json-with-dynamic-key-and-value-in-android). You have schema lik: `dynamic-key`->`dynamic-key`->`array` and it could look strange but it is pretty normal structure which we can handle using `Map`s and `List`s. — Michał Ziober, Feb 22 '19 at 01:07
`Jackson` was exactly what I was looking for. Thanks for the suggestion! — raphattack, Feb 23 '19 at 22:16

Parse JSON into Object

1 Answers1