CODEWITHSUNDEEP
javajsonparsinggson
Sami
Sami

Reputation: 93

Using GSON to parse Json into a JAVA Object where the Json Elements may vary

I am trying to parse a JSON .txt file into a JAVA object using GSON. The JSON file has the following structure:

    {
        "event0" : {
        "a" : "abc",
        "b" : "def"
        },

        "event1" : {
        "a" : "ghi",
        "b" : "jkl",
        "c" : "mno"
        }
    }

I have read the text file into a String called dataStr. I want to use the fromJson method to capture the events into the following JAVA class:

public class Event {
    private String a;
    private String b;
    private String c;

    public Event() {}

}

The problem is that the JSON might have one extra field "c" in some of the elements. I want to parse all the events into Event class objects, and for the cases where there is no "c" field, I want to make it null or zero in my object. It is not known beforehand which of the elements will have the "c" field.

Specifically, I was not able to figure out how to handle one extra field in some of the JSON elements. I want to do something along the lines of:

Gson gson = new Gson();
ArrayList<Event> events = gson.fromJson(dataStr, Event.class);

But I am stuck with first, how to iterate over the events in the Json file, and secondly, how to handle some occasional missing fields into the same Event object. I would really appreciate a kick in the right direction. Thank you all.

I am fairly new to JSON parsing, and might have missed something in the following answers:

Using Gson to convert Json into Java Object

Mapping JSON into POJO using Gson

Using gson to parse json to java object

Parse JSON into a java object

How to parse a json file into a java POJO class using GSON

Upvotes: 3

Views: 5750

Answers (4)

shusheng007
shusheng007

Reputation: 389

Try the below code snippet:

Gson gson = new Gson();
ArrayList<Event> events = gson.fromJson(dataStr, new TypeToken<ArrayList<Event>>(){}.getType());

In the source code of Gson has a very clear explain enter image description here

Upvotes: 0

StefanE
StefanE

Reputation: 7630

First I believe your JSON should look like this:

{
  "events": [
    {
      "name": "event0",
      "a": "abc",
      "b": "def"
    },
    {
      "name": "event1",
      "a": "abc",
      "b": "def",
      "c": "mno"
    }
  ]
}

This will need two classes for your model:

public List<Event> events = null;

public class Event {
public String name;
public String a;
public String b;
public String c;

}

And then then with GSON 

Events events = gson.fromJson(jsonData, Events.class);

Also I recommend to always use an online validator for JSON so you are sure your JSON structure is correct before coding against it. https://jsonlint.com/ Or for formate the JSON: http://jsonprettyprint.com/

Also this website can create the Java classes for you from either a JSON Schema or by using an example file. http://www.jsonschema2pojo.org/

Upvotes: 0

Ashish Lohia
Ashish Lohia

Reputation: 289

I'm not sure if I understood your question right. As per my understanding, you are trying to convert a json object with an extra field which is not available in the java class. Frankly, I don't understand why you want that or if it's possible to start with. You can have a workaround by converting the json to Map.

Map map = gson.fromJson(jsonString, Map.class);

Upvotes: 3

Gozus19
Gozus19

Reputation: 165

Gson automatically do that for you.

So, if you have a class "Alpha" with 3 fields ("a", "b" and "c") and you try to work on a json object that has 2 fields with names that match with Alpha's "a" and "b", Gson will fill "a" and "b" with json file's value and "c" will automatically set as null.

So, in your case, if you write this:

ArrayList<Event> events = gson.fromJson(dataStr, Event.class);

And in your json there are events with only 2 fields (that match with any Event's class fields) and events with all fields set, you will get a list of Events with no errors. Maybe you'll get some fields null, but the code will work.

I hope to be helpful! Ask for further informations, if you want to!

EDIT

Note that your json file has not to be .txt but .json instead!

Upvotes: 0

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