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How to get value for particular key only without reading whole JSON file in Java?

I need some information for reading a JSON file using Java. I have a sample data in abc.json file as following:
{
"id":"abc",
"name":"abc",
"otherInfo": {},
"personalData:[{}]
}
I am reading this file as following:
JSONParser parser = new JSONParser();
Object obj = parser.parse(new FileReader("...")); //the location of the file
JSONObject jsonObject = (JSONObject) obj;
JSONArray numbers = (JSONArray) jsonObject.get("personalData");
My question is will this above code read the whole file and then give data for ("personalData") key or will just fetch the data for particular key ("personalData"). And, if it fetches the entire's file data, is there any way to just fetch the value of a particular key without reading the whole file?

Reading json file and updating

Im trying to understand the procedure to do what the title says.
Im doing this in java with Gson dependency.
I am getting information from another service I use, in JSON format. So I want to get that info, put some additional info in there (like date/time) and use it afterwards for searching purposes.
The procedure is :
Get the JSON info (lets say "id") and add it to the JSON file you have
Add more info to that JSON file (lets say "Date and time of upload")
Finally, save that updated JSON file
So I read the file:
JsonReader reader = new JsonReader(new FileReader(filename));
Do I have now to convert it to string, and then update the string, so I can finally write it back to json?
If it doesn't exist, I create an empty file and then, can I update it with Json/Gson data? or do I have to create a Json File?
try {
File jsonFile = new File("C:\\uploads\\datasets");
if (jsonFile.createNewFile()){
System.out.println("File is created!");
}else{
System.out.println("File already exists.");
}
} catch (IOException e) {
e.printStackTrace();
}
Excuse any newbie/stupid mistakes I've probably made, I'm trying to understand JSON. Actually, the philosophy behind it.

JSON stands for JavaScript Object Notation and it's nothing more than a way to format data.
Taken from here:
JSON is built on two structures:
A collection of name/value pairs. In various languages, this is
realized as an object, record, struct, dictionary, hash table, keyed
list, or associative array.
An ordered list of values. In most
languages, this is realized as an array, vector, list, or sequence.
To address your questions:
Get the JSON info (lets say "id") and add it to the JSON file you have
JsonReader reader = new JsonReader(new FileReader(inputFilename));
reader.beginArray();
reader.beginObject();
long id = -1;
while (reader.hasNext()) {
String value = reader.nextName();
if (value.equals("id")) {
id = reader.nextLong();
} else {
reader.skipValue();
}
reader.endObject();
reader.endArray();
Add more info to that JSON file (lets say "Date and time of upload")
This will get the format in YYYY.MM.DD-HH.MM.SS
String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(Calendar.getInstance().getTime());
Finally, save that updated JSON file
Create a JsonWriter.
JsonWriter writer = new JsonWriter(new FileWriter(outputFilename));
writer.beginArray();
writer.beginObject();
writer.name("id").value(id);
writer.name("timestamp").value(timestamp);
writer.endObject();
writer.endArray();
You can read more about JsonReader and JsonWriter here and here.

Loop through json file appending key value to List

I am trying to loop through a JSON file and append the value each time to patientList. So far I believe I have done the hard part, however the simplest part seems to be taking a lot of time, that is appending the values to patientList. My getJsonFile method gets the path of the JSON file. The format of the JSON file is below. I am able to print jsonArray so I know I am good up to that point, but lost after that.
Json file.
[{"patient":1},{"patient":2},{"patient":3},{"patient":4},{"patient":5},{"patient":6},{"patient":7},{"patient":8},{"patient":9}]
getJsonFile method.
private List<Integer> getJsonFile(String path)
{
List<Integer> patientList = new ArrayList<>();
try (FileReader reader = new FileReader(path))
{
JSONParser jsonParser = new JSONParser();
JSONArray jsonArray = (JSONArray)jsonParser.parse(reader);
System.out.println(jsonArray);
// Update patientList
for (int i = 0; i < jsonArray.size(); i++ )
{
patientList.add(jsonArray(i));
}
}
catch(IOException | ParseException | NullPointerException e)
{
e.printStackTrace();
}
return patientList;
}

Your JSONArray contains objects: {"patient":1}
So you could not add patientList.add(jsonArray(i));
You have to access the int value inside that object:
JSONObject patient = jsonArray.getJsonObject(i);
patientList.add(patient.getInt("patient");
Edit
Well. You are using the simple-json library with quite limit feature and outdated. In this case you have to cast the data yourself:
JSONObject patient = (JSONObject)jsonArray.get(i);
patientList.add((Integer)patient.get("patient");
I recommend you remove this lib and use existing JSON feature of Java. If you want more advance feature, Jackson/GSon is the library to use.

How to change some values in a .JSON file and then write it back while keeping the JSON formatting ? (Java)

The JSON example file consists of:
{
"1st_key": "value1",
"2nd_key": "value2",
"object_keys": {
"obj_1st": "value1",
"obj_2nd": "value2",
"obj_3rd": "value3",
}
}
I read the JSON file into a String with this StringBuilder method, in order to add the newlines into the string itself. So the String looks exactly like the JSON file above.
public String getJsonContent(String fileName) {
StringBuilder result = new StringBuilder("");
File file = new File(fileName);
try (Scanner scanner = new Scanner(file)) {
while (scanner.hasNextLine()) {
String line = scanner.nextLine();
result.append(line).append("\n");
}
scanner.close();
} catch (IOException e) {
e.printStackTrace();
}
return result.toString();
}
Then I translate the JSON file into an Object using MongoDB API (with DBObject, BasicDBObject and util.JSON) and I call out the Object section I need to change, which is 'object_keys':
File jsonFile = new File(C:\\example.json);
String jsonString = getJsonContent(jsonFile.getAbsolutePath());
DBObject jsonObject = (DBObject)JSON.parse(jsonString);
BasicDBObject objectKeys = (BasicDBObject) jsonObject.get("object_keys");
Now I can write new values into the Object using the PUT method like this:
objectKeys.put("obj_1st","NEW_VALUE1");
objectKeys.put("obj_2nd","NEW_VALUE2");
objectKeys.put("obj_3rd","NEW_VALUE3");
! This following part not needed, check out my answer below.
After I have changed the object, I need to write it back into the json file, so I need to translate the Object into a String. There are two methods to do this, either one works.
String newJSON = jsonObject.toString();
or
String newJSON = JSON.serialize(jsonObject);
Then I write the content back into the file using PrintWriter
PrintWriter writer = new PrintWriter(C:\\example.json)
writer.print(newJSON);
writer.close();
The problem I am facing now is that the String that is written is in a single line with no formatting whatosever. Somewhere along the way it lost all the newlines. So it basically looks like this:
{"1st_key": "value1","2nd_key": "value2","object_keys": { "obj_1st": "NEW_VALUE1","obj_2nd": "NEW_VALUE2","obj_3rd": "NEW_VALUE3", }}
I need to write the JSON file back in the same format as shown in the beginning, keeping all the tabulation, spaces etc.
Is this possible somehow ?

When you want something formatted the way you said it is addressed as writing to a file in a pretty/beautiful way. For example: Output beautiful json. A quick search on google found what i believe to solve your problem.
Solution

You're going to have to use a json parser of some sort. I personally prefer org.json and would recommend it if you are manipulating the json data, but you may also like json-io which is really good for json serialization with no external dependencies.
With json-io, it's as simple as
String formattedJson = JsonWriter.formatJson(jsonObject.toString())
With org.json, you simply pass an int to the toString method.

Thanks Saraiva, I found a surprisingly simple solution by Googling around with the words 'pretty printing JSON' and used the Google GSON library. I downloaded the .jar and added it to my project in Eclipse.
These are the new imports I needed:
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
Since I already had the JSON Object (jsonObject) readily available from my previous code, I only needed to add two new lines:
Gson gson = new GsonBuilder().setPrettyPrinting().create();
String newJSON = gson.toJson(jsonObject);
Now when I use writer.print(newJSON); it will write the JSON in the right format, beautifully formatted and indented.

Parse ~1 MB JSON on Android very slow

I have an approximately 1MB JSON file stored in my assets folder that I need to load in my app every time it runs. I find that the built-in JSON parser (org.json) parses the file very slowly, however once it's parsed, I can access and manipulate the data very quickly. I've counted out as many as 7 or 8 seconds from the moment I click on the app to the moment the Activity1 is brought up, but just a few milliseconds to go from Activity1 to Activity2 which depends on data processed from the data loaded in Activity1.
I'm reading the file into memory and parsing it using:
String jsonString = readFileToMemory(myFilename)
JSONArray array = new JSONArray(jsonString);
where readFileToMemory(String) looks like this:
private String readFileToMemory(String filename) {
StringBuilder data = new StringBuilder();
BufferedReader reader = null;
try {
InputStream stream = myContext.getAssets().open(filename);
reader = new BufferedReader(new InputStreamReader(stream, "UTF-8"));
int result = 0;
do
{
char[] chunk = new char[512];
result = reader.read(chunk);
data.append(chunk);
}
while(result != -1);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
return data.toString();
}
Does anyone have any suggestions to how I can speed up the initial loading and parsing of the data? Should I perhaps mask the whole process behind a loading screen?

JSONObject -- the one from json.org, is the simplest API to use to parse JSON. However it comes with a cost -- performace. I have done extensive experiments with JSONObject, Gson and Jackson. It seems no matter what you do, JSONObject (and hence JSONArray) will be the slowest. Please switch to Jackson or Gson.
Here is the relative performance of the two
(fastest) Jackson > Gson >> JSONObject (slowest)
Refer:
- Jackson
- Gson

You should make an SQLite table to store the data and move it from JSON to SQL the first time the app runs. As an added benefit, this makes the data easier to search through and makes it possible for you to modify the data from within the app.