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Gson streaming API with different types of objects according to value of JSON field

I'm trying to parse the Wikidata JSON dump using the Gson streaming API, since the file is around 70GB of json. The overall structure of the file is as follows:
[
{"type":"item",... other fields ...},
{"type":"property",... other fields ...},
.....
]
It is an array of objects in which each object can be of type item or property and I would like to instantiate a different class (namely I have a corresponding Item and Property class in my Java code) according to the object that I encounter.
Basically, I'd like to look at the type field and then parse the following JSON accordingly. Since the JsonReader doesn't seem to provide a getNextJsonObject() or similar function, is there a way to do this besides preprocessing the whole file and splitting the entries into two separate ones? The file is so big that I'd like to avoid the extra preprocessing step when I could do everything on the fly.

I actually found a very easy solution after a bit of thinking. The Gson API provides the method:
Gson.fromJson(JsonReader reader, Class class)
This will read the next object from the reader and deserialize to the class you pass as parameter. Since in my case I don't know which class to serialize to I can do the following:
JsonObject asd = gson.fromJson(reader, JsonObject.class);
if (asd.get("type").getAsString().equals("item")) {
// Instantiate item
} else {
// Instantiate property
}

Gson: fields that sometimes are strings and others arrays

I'm parsing json with Gson but I'm struggling with the data I'm getting. This is part of an API out of my control (openFDA) so changing that might not be an option.
Here's the json I'm strugling with: https://api.fda.gov/device/event.json?search=device.generic_name:generator&limit=10
There are some fields that are not consistent, for example remedial_action. Sometimes it comes out like this:
"remedial_action": [
"Recall"
]
and in other results like this:
"remedial_action": ""
So it's either an array or a plain string. Is there a way to handle this? If not possible in Gson, any other json parsing library that can help?
I created my pojos here in case someone needs the code. There are a few files created from that and didn't want to spam them here. I can add them if needed.
Update: The bug has been confirmed and it's scheduled for a fix.

It is possible through GSON, by using a TypeAdapter.
Here are the initial steps I would use to do that:
Create a POJO that contains the array and the String. Let's call it RemedialAction.
In your original POJO, create an attribute of the new class.
Create a class that extends TypeAdapter<RemedialAction>.
Override the read() and write() methods and create the logic in them.
That should be a little hard to parse, though. Read this tutorial for more information.
Note: you can customize getRemedialAction() to give you only the valid return -- array or String.

Automatic Conversion of JSON to Java Object

I was wondering if there is any way or library which allows the automatic conversion of json strings to java objects ?
i.e. similar functionality provided by Backbone for javascript.
I am fully aware you can do it step by step for each field using something like:
JSONObject json = new JSONObject(jsonString);
String body = json.get("body").toString;
However I have many json files to convert and manually typing out the code for ever object I need seems tedious. Is there anyway to do it autonomously ?
I.e. provide an object template which can be populated regardless of it's fields ?

Most frameworks that use ajax allow you to set a config that will do this for you.

MyClass a = Gson().fromJson(jsonString, MyClass.class)

Java String to JSON and then JSON to array

I have the following String code and I want to convert to json and iterate through items and put them into an arraylist in Java.
I also have a class called Users whith its attributes and getters/setters(id, nick, age, online, avatar)how should i do:
code:
//this is the real code
String a = "{"id":"1","nick":"jhon","age":20,"online":1,"avatar":"http:\/\/www.example.com\/image.jpeg"},{"id":"2","nick":"mike","age":45,"online":0,"avatar":"http:\/\/www.example.com\/image.jpeg"},{"id":"3","nick":"carl","age":12,"online":1,"avatar":"http:\/\/www.example.com/image.jpeg"},{"id":"4","nick":"ana","age":22,"online":0,"avatar":"http:\/\/www.example.com\/image.jpeg"}";
//this is what i want to do
String a = real code sample;
Json b = a.toJson; // something like this
Arraylist<User> list = new Arraylist<User>();
for each b{
list.add( new user(b.getId(),b.getNick()....));
}
i want to do something like that and of course the code is an example and here it's not well written.

You may prefer to use Gson library which has good documentation for how to use. You can easly parse objects to JSON and JSON to object. So it may be a better alternative for your own parser.
It also supports Arrays and Collections.

I think you need to validate you array first on JSONLINT

Jackson is quite nice for this sort of thing - I've been using it for some time with no problems. I've heard nice things about Gson as well, but had no reason to switch from Jackson. Xstream also can work with Json.
Instead of working with a special "JSON" object, Jackson (and I believe Gson as well) stuff the JSON values into a POJO. You use the annotations to customize how the values are read and written.
FWIW, JSON processing is easy with a library like this, but also something you don't want to do much - it's relatively expensive in terms of CPU. It's not that the libs are slow - that kind of text processing is a lot of work.

How to pass an array of objects from javascript to Java

Hello Im trying to pass an array of objects from javascript to java , but how can this be done..??
I've found in some posts that they do this using a hidden input. Is this the only way?
I'm a bit confused. Please tell me what do I need to do to pass my array to the server? and which javascript files and jars do I need to add?
Thanks in advance.

You seem to want a completely baked-in solution. Not sure I can provide that, but here's what I'd do.
Indeed use a hidden input field in a form, where the value of the field is a valid JSON string. Send the form to your server, and in your servlet use a JSON Java library to parse the JSON string.
Here json-lib, gson or Jackson would do. In your case, I'd say json-lib would seem the easiest to use.
To generate the JSON string on the client-side, either use a framework or custom solution. For instance, jQuery has a serialize() function to serialize a form's fields to a JSON object directly, which you can then convert to string. Other frameworks provide similar functions.
To learn more about JSON, be sure to read the JSON Wikipedia entry and to visit the official JSON page (which also gives you a Java implementation of the JSON data-interchange format, though maybe not the most efficient one for processing a lot of data). To make sure your generated JSON is valid, you can use JSONLint.

If the objects are simple enough, you can encode your array as a JSON string. Java has libraries to encode and decode JSON.