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How do I create a json-string from object? – It returns an empty "[]"?

I'm trying to create make a method, that returns a json-string filled with sample-data. I have created a data-constructor class but when I when I create a data-object and afterwards print it, it for some reason returns an empty json: "[]"?
What am I missing here? Why doesn't it return the data-object I just created?
Here is my main class:
public class SimulatedDevice {
public static void printObject(Object object) {
Gson gson = new GsonBuilder().setPrettyPrinting().create();
System.out.println(gson.toJson(object));
}
public static class TelemetryDataPoint {
public String CreateTelemetryDataPoint() {
ArrayList<Data.TrendData> trendData = new ArrayList<>();
trendData.add(new Data.TrendData("Building1", "2018-08-28T01:03:02.997301Z", 2, "occupants", "int"));
Data data = new Data("B1", "0", "0", trendData);
printObject(data);
String json = new Gson().toJson(data);
return json;
}
}
}
This is my data constructor:
package com.microsoft.docs.iothub.samples;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import java.sql.Time;
import java.util.ArrayList;
import java.util.List;
public class Data extends ArrayList {
String Building;
String Floor;
String Zone;
ArrayList<TrendData> Trend;
public Data(String Building, String Floor, String Zone, ArrayList<TrendData> Trend) {
this.Building = Building;
this.Floor = Floor;
this.Zone = Zone;
this.Trend = Trend;
}
public static class TrendData {
String PointId;
String Timestamp;
int Value;
String Type;
String Unit;
public TrendData(String PointId, String Timestamp, int Value, String Type, String Unit) {
this.PointId = PointId;
this.Timestamp = Timestamp;
this.Value = Value;
this.Type = Type;
this.Unit = Unit;
}
}

If you remove 'extends ArrayList' from Data declaration it will work fine. I have not debugged it enough to figure out why Gson does not like the base class being ArrayList.
Here is a possible explanation: Trouble with Gson serializing an ArrayList of POJO's

Pojo having Clob type data, unable to convert Json to Object type using GSON library

I am POJO having Clob type variable. I am converting POJO type json to the POJO type Java Object using GSON library. But unable to convert. It's giving error like Unable to invoke no-args constructor for interface java.sql.Clob
My POJO Class
import java.sql.Clob;
public class MyPojo {
private String name;
private Clob message;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Clob getMessage() {
return message;
}
public void setMessage(Clob message) {
this.message = message;
}
}
Conversion class
import java.text.ParseException;
import org.json.JSONException;
import com.google.gson.Gson;
public class SaveClobType {
public static void main(String[] args) throws ParseException, JSONException {
Gson gson = new Gson();
String jsonString = "{\"name\" : \"Country\", \"message\" : \"Hello country\"}";
MyPojo obj = gson.fromJson(jsonString, MyPojo.class);
System.out.println("Object converted successfully");
}
}
Please anyone can help me for this. Thanks!

Custom Conversion from Java Object to JSON Object

I have the following code
Gson gson = new Gson();
String json = gson.toJson(criteria.list()); // list is passed by Hibernate
The result would be something like this:
{creationTime:0, enabled:true, id:1, loginDuration:0, online:false, userName:someone}
I would like to add new attribute (DT_RowId which has the same value as id) within the JSON response. The end result should be like this:
{creationTime:0, enabled:true, id:1, loginDuration:0, online:false, userName:someone, DT_RowId=1}
UPDATED
I created a field with #Transient annotation on the entity in order to solve this issue.
...
#Transient
private long DT_RowId;
public void setId(long id) {
this.id = id;
this.DT_RowId=id;
}
...
However the setId function was never been called. Can someone enlighten me on this ?

GSON won't call your getters and setters. It accesses member vars directly via reflection. To accomplish what you are trying to do, you will need to use a GSON custom serializer/deserializer. The GSON docs on custom serializers/deserializers provide some examples for how to do this.
Here is a working example with a passing JUnit test that demonstrates how to do it:
Entity.java
public class Entity {
protected long creationTime;
protected boolean enabled;
protected long id;
protected long loginDuration;
protected boolean online;
protected String userName;
protected long DT_RowId;
}
EntityJsonSerializer.java
import java.lang.reflect.Type;
import com.google.gson.*;
public class EntityJsonSerializer implements JsonSerializer<Entity> {
#Override
public JsonElement serialize(Entity entity, Type typeOfSrc, JsonSerializationContext context) {
entity.DT_RowId = entity.id;
Gson gson = new Gson();
return gson.toJsonTree(entity);
}
}
JSONTest.java
import static org.junit.Assert.*;
import org.junit.Test;
import com.google.gson.*;
public class JSONTest {
#Test
public final void testSerializeWithDTRowId() {
Entity entity = new Entity();
entity.creationTime = 0;
entity.enabled = true;
entity.id = 1;
entity.loginDuration = 0;
entity.online = false;
entity.userName = "someone";
GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(Entity.class, new EntityJsonSerializer());
Gson gson = builder.create();
String json = gson.toJson(entity);
String expectedJson = "{\"creationTime\":0,\"enabled\":true,\"id\":1,\"loginDuration\":0,\"online\":false,\"userName\":\"someone\",\"DT_RowId\":1}";
assertEquals(expectedJson, json);
}
}

Using gson to parse json to java object

I want to parse a json data into a java object, then post it to a .jsp to then turn it back into a json object so I can put it into a javascript array.
Here is an example of the data I want to parse:
"LoanList" = [ {"Loan" : "One","Lat" : "35.65365", "Lng" : "123.1331" , "Bal" : "4565" , "Address" : "Address 1" , "Delinquency" : "True')]
After reading about it, I decided to use gson to parse the data: After reading other questions about this topic, I wrote two classes.
class1.java
import java.util.ArrayList;
public class JSON1 {
ArrayList<innerData> Loanlst;
public ArrayList<innerData> getLoanlst() {
return Loanlst;
}
public void setLoanlst(ArrayList<innerData> Loanlst) {
this.Loanlst = Loanlst;
}
}
class2.java
import java.math.*;
public class innerData {
public String loan;
public BigDecimal lat;
public BigDecimal lng;
public BigDecimal bal;
public String addrs;
public String delinq;
public String getLoan() {
return loan;
}
public void setLoan(String loan){
this.loan = loan;
}
public BigDecimal getLat() {
return lat;
}
public void setLat(BigDecimal lat){
this.lat = lat;
}
public BigDecimal getLng() {
return lng;
}
public void setLng(BigDecimal lng){
this.lng = lng;
}
public BigDecimal getBal() {
return bal;
}
public void setBal(BigDecimal bal){
this.bal = bal;
}
public String getAddrs() {
return addrs;
}
public void setAddrs(String addrs){
this.addrs = addrs;
}
public String getDelinq() {
return delinq;
}
public void setDelinq(String delinq){
this.delinq = delinq;
}
}
Here is where I am stuck, Where do I create my new Gson() to parse the data before I send it?

Using your class you should be able to do something like:
InnerData obj = new InnerData(...);//fill in with data
Gson gson = new Gson();
// convert java object to JSON format,
// and returned as JSON formatted string
String json = gson.toJson(obj);
check an example here and here
Tip: try creating a main class to run it separately.

try this:
JsonParser parser = new JsonParser();
JsonObject jsonObject=parser.parse("your string").getAsJsonObject();
also if it is a jsonArray ,change getAsJsonObject to getAsJsonArray
also jackson offers an easy to to parser string to json
you can search jackson apache

How to expose a method using GSon?

Using Play Framework, I serialize my models via GSON. I specify which fields are exposed and which aren't.
This works great but I'd also like to #expose method too. Of course, this is too simple.
How can I do it ?
Thanks for your help !
public class Account extends Model {
#Expose
public String username;
#Expose
public String email;
public String password;
#Expose // Of course, this don't work
public String getEncodedPassword() {
// ...
}
}

The best solution I came with this problem was to make a dedicated serializer :
public class AccountSerializer implements JsonSerializer<Account> {
#Override
public JsonElement serialize(Account account, Type type, JsonSerializationContext context) {
JsonObject root = new JsonObject();
root.addProperty("id", account.id);
root.addProperty("email", account.email);
root.addProperty("encodedPassword", account.getEncodedPassword());
return root;
}
}
And to use it like this in my view:
GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(Account.class, new AccountSerializer());
Gson parser = gson.create();
renderJSON(parser.toJson(json));
But having #Expose working for a method would be great: it would avoid making a serializer just for showing methods!

Check out Gson on Fire: https://github.com/julman99/gson-fire
It's a library I made that extends Gson to handle cases like exposing method, results Post-serialization, Post-deserialization and many other things that I've needed over time with Gson.
This library is used in production in our company Contactive (http://goo.gl/yueXZ3), on both Android and the Java Backend

Gson's #Expose seem to only be supported on fields. There is an issue registered on this: #Expose should be used with methods.

Couple different options based on Cyril's answer:
Custom serializer with a shortcut:
public static class Sample
{
String firstName = "John";
String lastName = "Doe";
public String getFullName()
{
return firstName + " " + lastName;
}
}
public static class SampleSerializer implements JsonSerializer<Sample>
{
public JsonElement serialize(Sample src, Type typeOfSrc, JsonSerializationContext context)
{
JsonObject tree = (JsonObject)new Gson().toJsonTree(src);
tree.addProperty("fullName", src.getFullName());
return tree;
}
}
public static void main(String[] args) throws Exception
{
GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(Sample.class, new SampleSerializer());
Gson parser = gson.create();
System.out.println(parser.toJson(new Sample()));
}
-OR-
Annotation based serializer
public static class Sample
{
String firstName = "John";
String lastName = "Doe";
#ExposeMethod
public String getFullName()
{
return firstName + " " + lastName;
}
}
public static class MethodSerializer implements JsonSerializer<Object>
{
public JsonElement serialize(Object src, Type typeOfSrc, JsonSerializationContext context)
{
Gson gson = new Gson();
JsonObject tree = (JsonObject)gson.toJsonTree(src);
try
{
PropertyDescriptor[] properties = Introspector.getBeanInfo(src.getClass()).getPropertyDescriptors();
for (PropertyDescriptor property : properties)
{
if (property.getReadMethod().getAnnotation(ExposeMethod.class) != null)
{
Object result = property.getReadMethod().invoke(src, (Object[])null);
tree.add(property.getName(), gson.toJsonTree(result));
}
}
}
catch (Exception ex)
{
ex.printStackTrace();
}
return tree;
}
}
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.METHOD) //can use in method only.
public static #interface ExposeMethod {}
public static void main(String[] args) throws Exception
{
GsonBuilder gson = new GsonBuilder();
gson.registerTypeAdapter(Sample.class, new MethodSerializer());
Gson parser = gson.create();
System.out.println(parser.toJson(new Sample()));
}