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JSON-to-JSON transformation: possible approaches

Sometimes there’s a need to perform a same-format transformation from one structure into another.
Now before you start with the implementation, you first need the theoretical basis. therefore my question to you is what possible approaches for the transformation are possible for JSON to JSON?

If you are talking about a JSON string: convert into a HashMap, do what you have to do then convert back to JSON. You can use a library such as JSON.simple
https://mkyong.com/java/json-simple-example-read-and-write-json/

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.

How to parse a JSON object, update it in place and then save it back to disk in Java?

What is the easiest to use JSON Java library to parse a JSON (It's structure may vary so I can't mapp it to a Java class as I seen multiple libraries do it) update some elements in an array in this JSON and then save it back to disk?
There are so many libraries for JSON in Java (Gson, Jackson, etc.) and so complex that seem like a total overkill for what i need as opposed to other programming languages.
What's the most straightforward one to use? (that maybe also has a few examples on how to do this)

I have had great success with Json-Simple.
You can check it out here.
I used it to parse 1.5 Million Twitter Streaming data (which is in JSON).
You may find some sample code here on my blog.

You can use java-json jar. The docs for this jar can be found here

I use net.sf.json to do this.
here is an example :
String fromFile="{\"he\",\"hello\"}" //read from file instead
JSONObject object=(JSONObject)JSONSerializer.toJSON( fromFile );
object.put("he", "hello2");
System.out.println(object);
output:
{"he":"hello2"}

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.

How to create a JSON document using Java?

I want to read data from the database, convert it to docs (JSON) using Java.
Thanks

GSON is a Java library from Google to convert Java objects to JSON. You can simply pass a Java object to the library function and it will return a JSON string.
Download: http://code.google.com/p/google-gson/
Example: http://www.mkyong.com/java/how-do-convert-java-object-to-from-json-format-gson-api/
I have used the library from http://www.json.org, but the whole thing seems to be tedious to me. GSON is simple to use IMHO.

I would use JSONObject class:
http://www.json.org/javadoc/org/json/JSONObject.html
or you can build the string.