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How to jsonify a object with new values?

I have some object with different atributes' types: String,Float,LocalDate and so on. To turn this object into a json String, I use (com.google.gson.Gson)
gson.toJson(object);
But now I want to add a String to it:
{
"warning" : "old",
--Jsonified Object--
}
How can I add specific key-values data into a object without doing boilerplate code or creating a new object that has a new attribute plus the old ones?

You can use com.google.gson.JsonObject class to add extra fields.
JsonObject jsonObject = (JsonObject)gson.toJsonTree(object, YourObjects.class);
jsonObject.addProperty("warning" , "old"); // add required extra fields
String json = gson.toJson(jsonObject); // create son string

The easiest way to do this without creating a new class or modify the current is to work with maps. Of course it still needs some boilerplate, but you can create a method where you do all this.
With Gson starting from the already generated json you can create a map, modify this map and then create the json again.
Gson gson = new Gson();
Map<String, Object> map = gson.fromJson(json, new TypeToken<Map<String, Object>>(){}.getType());
// here you process the map, e.g. put or remove keys
map.put("warning", "old");
String json = gson.toJson(myMap);
If you do not want to start from the created JSON I recomend to transform your object in a map with jackson object mapper and then process the map and create the json. You should use another library like jackson because Gson does not provice a way to transform a object to a map.

How to Deserialize JsonString to jsonformat in java

a:34:
{
s:2:\"id\";
i:14;
s:10:\"created_at\";
s:19:\"2017-02-20 17:09:01\";
s:10:\"updated_at\";
s:19:\"2017-11-01 08:30:43\";
s:11:\"id\";i:3;s:7:\"username\";
}
these is format i am getting, how to deserialize.
Any help thankyou

The google has a very good library for JSON serialization/deserialization, named
Gson.
If you have a json string and you want to deserialize, you can do it simply by calling:
new Gson().fromJson(yourJsonString, Model.class);
where Model is the model class, where you can map your json string. If you check the library documentation, you can find more interesting things.

Convert Json to a java object

What is the way to generate a Java object with get and set methods?

You should write a java bean with properties maching the JSON key's, from that point since you already have a reader its a simple as
YourObject obj = gson.fromJson(br, YourObject.class);
UPDATE
With respect to your comment, when you don't want or can't create a bean it usually boils down to parsing JSON to map. GSON (afaik) doesn't have a built-in for this, but its not hard to build a method that will traverse GSON's objects. You have an example in this blog
http://itsmyviewofthings.blogspot.it/2013/04/jsonconverter-code-that-converts-json.html
As you seem to be open to alternatives, take a look at Jackson as well (the two libs are the de-facto standard in JAVA).
With jackson you don't have to create a bean to support deserialization, e.g.
String json = "{\"id\":\"masterslave\"}";
Map<String,String> map = new HashMap<String,String>();
ObjectMapper mapper = new ObjectMapper();
//convert JSON string to Map
map = mapper.readValue(json,
new TypeReference<HashMap<String,String>>(){});

http://www.jsonschema2pojo.org/
That link helps generate the Java object format based on the GSON you feed in. Just make sure you set the settings exactly as you need it. As always, it's not a good idea to just copy-paste generated code, but it might be of help.

Json, Jackson and serialization

I've started to using Json with Jackson library and i found little problem.
I'm creating Json object:
ObjectMapper mapper = new ObjectMapper();
Map<String, Object> userInMap = new HashMap<String, Object>();
then i'm adding fields:
userInMap.put("user", "active");
userInMap.put("uuid", uuid);
And after all when im trying to output this object i have Json object but without ", i mean i supposed to have:
{"user":"active", "uuid":"lasdnfa"}
but i have:
{user:active, uuid:lasdnfa}
and another thing - i want to add this object to memcache, but before i do this, i have to serialize this object. How i can serialize Json object?
Thanks

If you are using toString() on your object, you might need your mapper to output the value this way :
System.out.println(mapper.writeValueAsString(userInMap)));

Constructing POJO out of JSON string with dynamic fields using Gson

I'm consuming a web service in my application that will return a list of ID's associated with a name. An example would look like this:
{
"6502": "News",
"6503": "Sports",
"6505": "Opinion",
"6501": "Arts",
"6506": "The Statement"
}
How would I construct a POJO for Gson to deserialize to when all of the fields are dynamic?

How about deserializing into a map?
Gson gson = new Gson();
Type mapType = new TypeToken<Map<String, String>>() {}.getType();
String json = "{'6502':'News','6503':'Sports','6505':'Opinion','6501':'Arts','6506':'The Statement'}";
Map<String, String> map = gson.fromJson(json, mapType);
Using a map sounds reasonable for me (as Java is statically typed). Even if this could work (maybe using JavaCompiler) - accessing the object would not probably be much different from accessing a map.

I don't know Gson that well but I suspect that's not possible. You'd have to know which fields are possible beforehand, although the fields might not be in the Json and thus be null.
You might be able to create classes at runtime by parsing the Json string, but I don't know whether that would be worth the hassle.
If everything is dynamic your best bet would be to deserialize the Json string to maps of strings or arrays etc. like other Json libraries do (I don't know whether Gson can do this as well, but the classes you need are commonly called JSONObject and JSONArray).
So your Json string above would then result in a Map<String, String>.