We Keep Coding

How to get a value which is a List from a JSONObject in Java

I have a following JSON:
{"data":["str1", "str2", "str3"]}
I want to get a List, i.e. ["str1", "str2", "str3"]
My code is:
JSONObject json = new JSONObject();
List list = new ArrayList();
...
// adding data in json
...
list = (List) json.get("data");
This is not working.

you can get this data as a JsonArray

You can customize a little bit of code like it
public static void main(String[] args) throws ParseException {
String data = "{\"data\":[\"str1\", \"str2\", \"str3\"]}";
JSONObject json = new JSONObject(
data);
JSONArray jasonArray = json.getJSONArray("data");
List list = new ArrayList();
int size = jasonArray.length();
int i = 0;
while (i < size) {
list.add(jasonArray.get(i));
i++;
}
System.out.println(list);
}

You wish to parse a JSON string using Java code. It is recommended to use a JSON library for Java. There are several. The below code uses Gson. There are many online examples such as Convert String to JsonObject with Gson. You should also familiarize yourself with the Gson API.
import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import java.util.ArrayList;
import java.util.List;
public class JsonList {
public static void main(String[] args) {
String json = "{\"data\":[\"str1\", \"str2\", \"str3\"]}";
JsonElement elem = JsonParser.parseString(json);
if (elem.isJsonObject()) {
JsonObject obj = elem.getAsJsonObject();
elem = obj.get("data");
if (elem.isJsonArray()) {
JsonArray arr = elem.getAsJsonArray();
List<String> list = new ArrayList<>();
int count = arr.size();
for (int i = 0; i < count; i++) {
elem = arr.get(i);
if (elem.isJsonPrimitive()) {
String str = elem.getAsString();
list.add(str);
}
}
System.out.println(list);
}
}
}
}
Running the above code gives the following output:
[str1, str2, str3]
There are other ways to convert the JsonArray to a List. The above is not the only way. As I wrote earlier, peruse the API documentation and search the Internet.

Behind the scenes, the JSONArray object stores the json data in an ArrayList<Object>, and it has a method called toList(). There's absolutely no need to loop through the JSONArray in order to set values in the array. The simpler code would look something like this
String data = "{\"data\":[\"str1\", \"str2\", \"str3\"]}";
JSONObject json = new JSONObject(data);
List<Object> list = json.getJSONArray("data").toList();
System.out.println(myList);
Note: This will create a list of generic Objects. The currently accepted answer doesn't define a type for the List, which is unsafe. It doesn't enforce type safety, and errors will occur at runtime instead of at compile time.
If you want to convert all of the inner objects to a String, you can do this by upcasting the List to an Object, and then casting it to a List<String>. I don't particularly recommend it, but it can be done like this. List<String> list = (List<String>) (Object) json.getJSONArray("data").toList();.
A better way of casting the value to a specific type would be via a stream to call the Object.toString() method.
List<String> list = json.getJSONArray("data").toList().stream().map(Object::toString).collect(Collectors.toList());
or, if you have a specific type you want to cast it to, you can use
List<MyObject> list = json.getJSONArray("data").toList().stream().map(jsonObject -> (MyObject) jsonObject).collect(Collectors.toList());
Finally, as others have pointed out, there are better libraries for dealing with json. Gson is a great library, however I personally prefer Jackson. They both offer similar resources, but I've found that Jackson's ObjectMapper is more customizable and more widely used.

Converting Arraylist<Arraylist<Integer>> to json

I am quite new to java and android so please keep that in mind.
I have an Arraylist of an Arraylist of integers. These integers are data from the GPS like Latitude, longtitude, speed at that moment and distance, added to an arraylist in OnLocationChanged and all these arraylists are then added to another arraylist.(sort of like a matrix or table)
example: [[timestamp,lat,long,distance_from_start,speed],[...],...] (all are integers)
I want to convert this Arraylist of arraylists so i can save it on the internal storage of my app for use in other activities ( like statistics of this data) and to upload it to a server. I have searched around quite a bit and found that converting an arraylist to json allows this and also makes it easier to create an SQL file of this data. The conversion of the arraylist to json seems easy enough but i can't find any examples of converting an arraylist of arraylists to json. So i dont know if the arraylists in the arraylist are converted to jsonarrays or whatever or if they will be usable and readable from the json file at all. If this is not possible, are there any other alternative ways of doing this?
Thanks a lot!

Use org.json.JsonArray library.
import java.util.ArrayList;
import java.util.List;
import org.json.JSONArray;
public class Test {
public static void main(String[] args) {
List<List<Integer >> list= new ArrayList<List<Integer>>();
List<Integer> list1=new ArrayList();
list1.add(10);
list1.add(20);
list.add(list1);
List<Integer> list2=new ArrayList();
list2.add(60);
list2.add(70);
list.add(list2);
JSONArray jsonArray= new JSONArray(list);
System.out.println(jsonArray);
}
}
output:
[[10,20],[60,70]]

You can use Gson from Google.
Your main functions are: toJson and fromJson.
From the javadoc:
toJson(Object src)
This method serializes the specified object into its equivalent Json representation.
fromJson(String json, Type typeOfT)
This method deserializes the specified Json into an object of the specified type.
For example:
(Serialization)
Gson gson = new Gson();
gson.toJson(1); ==> prints 1
gson.toJson("abcd"); ==> prints "abcd"
gson.toJson(new Long(10)); ==> prints 10
int[] values = { 1 };
gson.toJson(values); ==> prints [1]
(Deserialization)
int one = gson.fromJson("1", int.class);
Integer one = gson.fromJson("1", Integer.class);
Long one = gson.fromJson("1", Long.class);
Boolean false = gson.fromJson("false", Boolean.class);
String str = gson.fromJson("\"abc\"", String.class);
String anotherStr = gson.fromJson("[\"abc\"]", String.class);
Object Examples
class BagOfPrimitives {
private int value1 = 1;
private String value2 = "abc";
private transient int value3 = 3;
BagOfPrimitives() {
// no-args constructor
}
}
(Serialization)
BagOfPrimitives obj = new BagOfPrimitives();
Gson gson = new Gson();
String json = gson.toJson(obj);
==> json is {"value1":1,"value2":"abc"}
Note that you can not serialize objects with circular references since that will result in infinite recursion.
(Deserialization)
BagOfPrimitives obj2 = gson.fromJson(json, BagOfPrimitives.class);
==> obj2 is just like obj
List of Lists of Integers
List<List<Integer >> list = new ArrayList<List<Integer>>();
List<Integer> list1=new ArrayList();
list1.add(100);
list1.add(200);
list.add(list1);
List<Integer> list2=new ArrayList();
list2.add(700);
list2.add(800);
list.add(list2);
Gson gson = new Gson()
String json = gson.toJson(list);
System.out.println(json);

How to clear data from a JSON Array

I am working on a project where I have to clear all the data from a JSON array. There seems to be no method like jsonArray.clear(). Also tried jsonArray = new JSONArray(). That too didn't worked. Suggestions please

Just create a new JSONArray.
JSONArray otherJsonArray = new JSONArray();
Or iterate through the array and remove(int index) the indexes.
http://www.json.org/javadoc/org/json/JSONArray.html#remove(int)

Just put jsonArray = new JSONArray()

Creating a new one will work, unless you have passed it as a parameter to a method in which case you need to modify the referenced object as a new reference will not be seen by the calling method.
So if that is the case, do it backwards, that way you won't get your iterator exceeding bounds:
int startingLength = someJsonArray.length();
for (int i = startingLength - 1; i >= 0; i--) {
someJsonArray.remove(i);
}

And you use that
otherJsonArray is already existing then you use
JSONArray otherJsonArray = new JSONArray("[]");

We can use someJsonArray.pop(index) to remove require record.
We can use this code in loop to remove all records.

I have a situation where I want to remove all the entries from a JSONArray with key "Constants", which is an element in a JSONObject, creating a new JSONArray an assigning it does NOT clear the JSONArray, I have to iterate through the JSONArray and jsonArray.remove(i) on each of them, but there is a second method that works which involves removing the array element, in this case "Constants" completely from the JSONObject and re-adding it as a new JSONArray.
Hereis code with assignment of new array, which does not work, the JSONArray remained unchanged: (I tried both above suggestions for new JSONArray(); and new JSONArray("[]");
JSONObject jsonObj = new JSONObject(metadataOriginalJSON);
if (jsonObj.isJSONArray("Constants")) {
JSONArray constantsArray = jsonObj.getJSONArray("Constants");
constantsArray = new JSONArray();
metadataConstantsRemoved = jsonObj.toString();
}
Here is code for the iteration through the JSONArray which worked:
JSONObject jsonObj = new JSONObject(metadataOriginalJSON);
if (jsonObj.isJSONArray("Constants")) {
JSONArray constantsArray = jsonObj.getJSONArray("Constants");
int i = 0;
int arrayLenSanityCheckPreventEndlessLoop = constantsArray.length();
while (constantsArray.length() > 0 && i < arrayLenSanityCheckPreventEndlessLoop) {
constantsArray.remove(0);
i++;
}
metadataConstantsRemoved = jsonObj.toString();
}
The 2nd method that works by removing the entire JSONArray element and re-adding it to the JSONObject:
JSONObject jsonObj = new JSONObject(metadataOriginalJSON);
if (jsonObj.isJSONArray("Constants")) {
jsonObj.remove("Constants");
jsonObj.put("Constants", new JSONArray());
metadataConstantsRemoved = jsonObj.toString();
}

In Java, how can I combine two JSON arrays of objects?

I have several string each containing a JSON representation of an array of objects. Here's an example in code to illustrate, though this is not my actual code (the JSON strings are passed in):
String s1 = "[{name: "Bob", car: "Ford"},{name: "Mary", car: "Fiat"}]";
String s2 = "[{name: "Mack", car: "VW"},{name: "Steve", car: "Mercedes Benz"}]";
I need to combine those two JSON arrays into one large JSON array. I could treat this as a String manipulation problem and replace the inner end square brackets with commas but that's not particularly robust (though I am guaranteed to get valid JSON).
I'd rather treat these two Strings as JSON arrays and just add them together somehow. It's a great plan except I don't know the "somehow" part.
Does anyone know a solution in Java that doesn't require constructing Java Object representations of the JSON objects?
Thanks!

This code will take sourceArray (s2), and append it to the end of destinationArray (s1):
String s1 = "[{name: \"Bob\", car: \"Ford\"},{name: \"Mary\", car: \"Fiat\"}]";
String s2 = "[{name: \"Mack\", car: \"VW\"},{name: \"Steve\", car: \"Mercedes Benz\"}]";
JSONArray sourceArray = new JSONArray(s2);
JSONArray destinationArray = new JSONArray(s1);
for (int i = 0; i < sourceArray.length(); i++) {
destinationArray.put(sourceArray.getJSONObject(i));
}
String s3 = destinationArray.toString();

You really have only two choices: parse the JSON (which invariably would involve constructing the objects) or don't parse the JSON. Not parsing is going to be cheaper, of course.
At first glance your idea about treating it as a String-manipulation problem might sound fragile, but the more I think about it, the more it seems to make fine sense. For error detection you could easily confirm that you were really dealing with arrays by checking for the square brackets; after that, just stripping off the ending bracket, adding a comma, stripping off the beginning bracket, and adding the "tail" should work flawlessly. The only exception I can think of is if either array is empty, you should just return the other String unchanged; again, that's very easy to check for as a String.
I really don't think there's any reason to make it more complex than that.

I used this code for Combine two Json Array.
String s1 = "[{name: \"Bob\", car: \"Ford\"},{name: \"Mary\", car: \"Fiat\"}]";
String s2 = "[{name: \"Mack\", car: \"VW\"},{name: \"Steve\", car: \"Mercedes Benz\"}]";
String s3=new String("");
s1=s1.substring(s1.indexOf("[")+1, s1.lastIndexOf("]"));
s2=s2.substring(s2.indexOf("[")+1, s2.lastIndexOf("]"));
s3="["+s1+","+s2+"]";
System.out.println(s3);

And here is my solution, You may want to merge more than two arrays :
Java version:
public static JSONArray mergeMultiJsonArray(JSONArray... arrays) {
JSONArray outArray = new JSONArray();
for (JSONArray array : arrays)
for (int i = 0; i < array.length(); i++)
outArray.put(array.optJSONObject(i));
return outArray;
}
Kotlin version:
fun mergeMultiJsonArray(vararg arrays: JSONArray): JSONArray {
val outArr = JSONArray()
for (array in arrays)
for (i in 0 until array.length())
outArray.put(array.optJSONObject(i))
return outArr
}

i use this code to append all the elements of a jsonArray to a common JsonArray.
public JSONArray getMergeJsonArrays(ArrayList<JSONArray> jsonArrays) throws JSONException
{
JSONArray MergedJsonArrays= new JSONArray();
for(JSONArray tmpArray:jsonArrays)
{
for(int i=0;i<tmpArray.length();i++)
{
MergedJsonArrays.put(tmpArray.get(i));
}
}
return MergedJsonArrays;
}

This function does the magic, adding multiples arrays returning one JSONArray with all elements
public static JSONArray JoinArrays(JSONArray... jsonArrays) {
JSONArray resultJSONArray = new JSONArray();
Arrays.stream(jsonArrays).forEach(jsonArray -> IntStream.range(0, jsonArray.length()).mapToObj(jsonArray::get).forEach(resultJSONArray::put));
return resultJSONArray;
}

Use Below Method pass all JSON array in ArrayList this method will return cumulative JsonArray
public JSONArray getMergeJson(ArrayList<JSONArray> xyz){
JSONArray result=null;
JSONObject obj= new JSONObject();
obj.put("key",result);
for(JSONArray tmp:patches){
for(int i=0;i<tmp.length();i++){
obj.append("key", tmp.getJSONObject(i)); ;
}
}
return obj.getJSONArray("key");
}

JSON Array iteration in Android/Java

I am building an android app that needs to download and synchronise with an online database, I am sending my query from the app to a php page which returns the relevant rows from a database in JSON format.
can someone please tell me the best way to iterate through a JSON array?
I receive an array of objects:
[{json object},{json object},{json object}]
What is the simplest piece of code I could use to access the JSONObjects in the array?
EDIT: now that I think of it the method I used to iterate the loop was:
for (String row: json){
id = row.getInt("id");
name = row.getString("name");
password = row.getString("password");
}
So I guess I had was somehow able to turn the returned Json into and iterable array. Any Ideas how I could achieve this?
I apologise for my vaguness but I had this working from an example I found on the web and have since been unable to find it.

I think this code is short and clear:
int id;
String name;
JSONArray array = new JSONArray(string_of_json_array);
for (int i = 0; i < array.length(); i++) {
JSONObject row = array.getJSONObject(i);
id = row.getInt("id");
name = row.getString("name");
}
Is that what you were looking for?

I have done it two different ways,
1.) make a Map
HashMap<String, String> applicationSettings = new HashMap<String,String>();
for(int i=0; i<settings.length(); i++){
String value = settings.getJSONObject(i).getString("value");
String name = settings.getJSONObject(i).getString("name");
applicationSettings.put(name, value);
}
2.) make a JSONArray of names
JSONArray names = json.names();
JSONArray values = json.toJSONArray(names);
for(int i=0; i<values.length(); i++){
if (names.getString(i).equals("description")){
setDescription(values.getString(i));
}
else if (names.getString(i).equals("expiryDate")){
String dateString = values.getString(i);
setExpiryDate(stringToDateHelper(dateString));
}
else if (names.getString(i).equals("id")){
setId(values.getLong(i));
}
else if (names.getString(i).equals("offerCode")){
setOfferCode(values.getString(i));
}
else if (names.getString(i).equals("startDate")){
String dateString = values.getString(i);
setStartDate(stringToDateHelper(dateString));
}
else if (names.getString(i).equals("title")){
setTitle(values.getString(i));
}
}

Unfortunately , JSONArray doesn't support foreach statements, like:
for(JSONObject someObj : someJsonArray) {
// do something about someObj
....
....
}

When I tried #vipw's suggestion, I was faced with this exception:
The method getJSONObject(int) is undefined for the type JSONArray
This worked for me instead:
int myJsonArraySize = myJsonArray.size();
for (int i = 0; i < myJsonArraySize; i++) {
JSONObject myJsonObject = (JSONObject) myJsonArray.get(i);
// Do whatever you have to do to myJsonObject...
}

If you're using the JSON.org Java implementation, which is open source, you can just make JSONArray implement the Iterable interface and add the following method to the class:
#Override
public Iterator iterator() {
return this.myArrayList.iterator();
}
This will make all instances of JSONArray iterable, meaning that the for (Object foo : bar) syntax will now work with it (note that foo has to be an Object, because JSONArrays do not have a declared type). All this works because the JSONArray class is backed by a simple ArrayList, which is already iterable. I imagine that other open source implementations would be just as easy to change.

On Arrays, look for:
JSONArray menuitemArray = popupObject.getJSONArray("menuitem");

You are using the same Cast object for every entry.
On each iteration you just changed the same object instead creating a new one.
This code should fix it:
JSONArray jCastArr = jObj.getJSONArray("abridged_cast");
ArrayList<Cast> castList= new ArrayList<Cast>();
for (int i=0; i < jCastArr.length(); i++) {
Cast person = new Cast(); // create a new object here
JSONObject jpersonObj = jCastArr.getJSONObject(i);
person.castId = (String) jpersonObj.getString("id");
person.castFullName = (String) jpersonObj.getString("name");
castList.add(person);
}
details.castList = castList;

While iterating over a JSON array (org.json.JSONArray, built into Android), watch out for null objects; for example, you may get "null" instead of a null string.
A check may look like:
s[i] = array.isNull(i) ? null : array.getString(i);