Converting Json Data to Java Object

Converting JSON data to Java object

I looked at Google's Gson as a potential JSON plugin. Can anyone offer some form of guidance as to how I can generate Java from this JSON string?

Google Gson supports generics and nested beans. The [] in JSON represents an array and should map to a Java collection such as List or just a plain Java array. The {} in JSON represents an object and should map to a Java Map or just some JavaBean class.

You have a JSON object with several properties of which the groups property represents an array of nested objects of the very same type. This can be parsed with Gson the following way:

package com.stackoverflow.q1688099;

import java.util.List;
import com.google.gson.Gson;

public class Test {

    public static void main(String... args) throws Exception {
        String json = 
            "{"
                + "'title': 'Computing and Information systems',"
                + "'id' : 1,"
                + "'children' : 'true',"
                + "'groups' : [{"
                    + "'title' : 'Level one CIS',"
                    + "'id' : 2,"
                    + "'children' : 'true',"
                    + "'groups' : [{"
                        + "'title' : 'Intro To Computing and Internet',"
                        + "'id' : 3,"
                        + "'children': 'false',"
                        + "'groups':[]"
                    + "}]" 
                + "}]"
            + "}";

        // Now do the magic.
        Data data = new Gson().fromJson(json, Data.class);

        // Show it.
        System.out.println(data);
    }

}

class Data {
    private String title;
    private Long id;
    private Boolean children;
    private List<Data> groups;

    public String getTitle() { return title; }
    public Long getId() { return id; }
    public Boolean getChildren() { return children; }
    public List<Data> getGroups() { return groups; }

    public void setTitle(String title) { this.title = title; }
    public void setId(Long id) { this.id = id; }
    public void setChildren(Boolean children) { this.children = children; }
    public void setGroups(List<Data> groups) { this.groups = groups; }
    
    public String toString() {
        return String.format("title:%s,id:%d,children:%s,groups:%s", title, id, children, groups);
    }
}

Fairly simple, isn't it? Just have a suitable JavaBean and call Gson#fromJson().

See also:

• Json.org - Introduction to JSON
• Gson User Guide - Introduction to Gson

How to convert the following json string to java object?

No need to go with GSON for this; Jackson can do either plain Maps/Lists:

ObjectMapper mapper = new ObjectMapper();
Map<String,Object> map = mapper.readValue(json, Map.class);

or more convenient JSON Tree:

JsonNode rootNode = mapper.readTree(json);

By the way, there is no reason why you could not actually create Java classes and do it (IMO) more conveniently:

public class Library {
  @JsonProperty("libraryname")
  public String name;

  @JsonProperty("mymusic")
  public List<Song> songs;
}
public class Song {
  @JsonProperty("Artist Name") public String artistName;
  @JsonProperty("Song Name") public String songName;
}

Library lib = mapper.readValue(jsonString, Library.class);

How to convert JSON to Java object?

user ObjectMapper

ObjectMapper mapper = new ObjectMapper();
Map<String,Object> map = mapper.readValue(json, Map.class);

Check This Link

How to convert JSON to Java object if I do not know all json fields before parsing?

There are three ways I've done this. The first is to do what you're suggesting - parse the JSON, check the type, and create the object. Be very careful with using a String parser as you may or may not have things like new lines. Instead, do something like:

ObjectMapper objectMapper = new ObjectMapper();
JsonNode jsonNode = objectMapper.readTree(eventString);
String eventType = jsonNode.get("event").asText();

if( eventType.equalsIgnoreCase("conversation_started")) {
    // create ConversationStarted object using something like:
    ConversationStarted conversationStarted = objectMapper.readValue( eventString, ConversationStarted.class );
}

This, of course, requires all classes to have a concrete POJO to allow for deserialization.

Another way is to do what many other programming languages do and have a key/value map. There are a few ways to do this. One is with the Jackson libraries:

Map<String, Object> map = objectMapper.readValue(eventString, new TypeReference<Map<String,Object>>(){});

Map<String, Object> user = (Map<String, Object>) map.get("user");

System.out.println( "conversation started - avatar is " + user.get("avatar"));

That way you can pass around the Map and extract as needed. Note that you still need to understand the structure of the JSON but you don't need to have a POJO for it.

Lastly is a variation on the second solution. Using JSONPath you can pull out what you need directly. Again you will want to first check out which type of event you have. Something like:

if( JsonPath.read(eventString, "$.event").equals("conversation_started") ) {
    String avatar = JsonPath.read(eventString, "$.user.avatar");
    System.out.println("conversation started - avatar is " + avatar);
 }

The last two methods require you to pull out values one at a time as shown. The first solution gives you a full object to work with. It is your call as to what works best in your environment.

How to convert JSON string into List of Java object?

You are asking Jackson to parse a StudentList. Tell it to parse a List (of students) instead. Since List is generic you will typically use a TypeReference

List<Student> participantJsonList = mapper.readValue(jsonString, new TypeReference<List<Student>>(){});

Converting a .json file into a Java object with a custom deserializer Jackson

The issue is with my Clothing class not having a constructor with empty parameters. I simply added another Constructor.

The issue with the additional unrecognized field "numberOfclothing" is due to my Closet and StyleBoard Classes both having "getters" for values that weren't part of a field but for the size of fields using the List type. I simply had to change the name of these methods to not include "get" as part of their name.

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