Extract an array of objects from json string and put them in ArrayList?

Question

I’ve a simple question, and was wondering if someone here could help me how to parse through a Json string and extract all the User and put it into a List?

Note the object key directly under Users is a random number which can change with every response!

{
    "_links": {},
    "count": {},
    "users": {
        "123321": { //*Is a random number which can be any number
            "_links": {},
            "user": {
                "id": "123321",
                "name": "...",
                "age": "...",
                "address": ""
                ..
            }
        },
        "456654": {
            "_links": {},
            "user": {
                "id": "456654",
                "name": "...",
                "age": "...",
                "address": ""
                ...
            }
        }
        ...
    },
    "page": {}
}

Ideally, it would be nice to utilize Jackson Json or Gson.

Also I would like to only construct User object with two fields (id & name) and ignore the rest of the fields!

Create a class mapping the structure of the JSON (at least what you're interested into), and use Jackson to read an instance of that class. Use a Map<String, SomeObjectWithAUserFiel> for the users member. — JB Nizet
– JB Nizet, Commented Nov 12, 2017 at 22:48
How do I ignore the fields that I’m not interested? Can you provide and example of this class and how to use Jackson to iteration over the tree and create this ArrayList please! — Simple-Solution
– Simple-Solution, Commented Nov 12, 2017 at 22:51
By not putting them into the class. You need to start experimenting by yourself. Extracting the values from a map is a matter of reading the javadoc of Map. — JB Nizet
– JB Nizet, Commented Nov 12, 2017 at 22:54

Henrik Aasted Sørensen · Accepted Answer · 2017-11-13 19:17:21Z

3

A Jackson-approach could look like this:

ObjectMapper om = new ObjectMapper();
JsonNode node = om.readTree(input);

List<JsonNode> users = node.findValues("user");

From here you can either retrieve the values directly from the JsonNode instances, e.g.:

for (JsonNode jsonNode : users) {
    System.out.println(jsonNode.get("name"));
    System.out.println(jsonNode.get("id"));
}

or convert to a POJO:

for (JsonNode user : users) {
    User pojo = om.treeToValue(user, User.class);
    System.out.println(pojo.id);
    System.out.println(pojo.name);
}

Assuming the existence of a POJO that looks something like the following:

@JsonIgnoreProperties(ignoreUnknown = true)
public class User {
    public String name, id;
}

edited Nov 13, 2017 at 19:17

answered Nov 13, 2017 at 16:45

Henrik Aasted Sørensen

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Henrik Aasted Sørensen · Accepted Answer · 2017-11-13 16:12:41Z

2

You have to parse the json...which can be done like -

JSONObject json=new JSONObject(response);
JSONObject json2=json.getJSONObject("users");
Iterator keys = json2.keys();

while(keys.hasNext()) {
    String key = (String)keys.next();
    JSONObject json3=json2.getJSONObject(key);
    ArrayList<User> list=new ArrayList<>();

    JSONObject userjson=json3.getJSONObject("user");    
    User user = new User();
    String id=userjson.getString("id");
    String name=userjson.getString("name");
    user.setId(id);
    user.setName(name);
    list.add(user);      
}

Hope it helps.. :)

edited Nov 13, 2017 at 16:12

Henrik Aasted Sørensen

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answered Nov 12, 2017 at 22:51

Ankit

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3 Comments

Ankit Over a year ago

No, the solution is not taking into account the random numbers.. This is most standard way of performing json parsing as far as i know..For 100 users it won't take time..I have implemented this in many standard apps having 5000 users.

Ankit Over a year ago

Check the code now..It will work most probably. I have parsed random number too by ignoring them. the list will be containing only the user object, i.e list of users as you want..And What else can be a more effective way to do this, since we have to save each user object in list..so somehow we will do so in the loop only..It won't take time..Using streams class will do the same thing and the complexity will be same.

Ankit Over a year ago

mvnrepository.com/artifact/org.json/json Mark this correct if it works.. :)

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