Question

The following code is working

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.Gson;

class DataObject<T> {

    private int data1 = 100;
    private String data2 = "hello";
DataObject child;
private List<String> list = new ArrayList<String>() {
  {
    add("String 1");
    add("String 2");
    add("String 3");
      }
    };
    private Map<String, DataObject> data=null;

    public DataObject(int i){
        this.data1 = i;
        this.data = new HashMap<String, DataObject>();
    }

    //getter and setter methods

    @Override
    public String toString() {
       return "DataObject [data1=" + data1 + ", data2=" + data2 + ", list="
    + list + "]";
    }
    public Map<String, DataObject> getData() {
        return data;
    }

    public void addData(final String key, DataObject value, Class<T> t) {
        data.put(key, value);
    }
}

public class test {



    /**
 * @param args
 */
public static void main(String[] args) {    
    DataObject obj = new DataObject(12);
    obj.child = new DataObject(25);
    obj.addData("myOtherData", new DataObject(32), DataObject.class);
        Gson gson = new Gson();
        System.out.println(gson.toJson(obj));
    }

}

output:

{"data1":12,"data2":"hello","child":{"data1":25,"data2":"hello","list":["String 1","String 2","String 3"],"data":{}},"list":["String 1","String 2","String 3"],"data":{"myOtherData":{"data1":32,"data2":"hello","list":["String 1","String 2","String 3"],"data":{}}}}

but I actually need to make the work with a generic

Map<String, Object> data

So this code:

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.Gson;

class DataObject<T> {

    private int data1 = 100;
    private String data2 = "hello";
DataObject child;
private List<String> list = new ArrayList<String>() {
  {
    add("String 1");
    add("String 2");
    add("String 3");
      }
    };
    private Map<String, Object> data=null;

    public DataObject(int i){
        this.data1 = i;
        this.data = new HashMap<String, Object>();
    }

    //getter and setter methods

    @Override
    public String toString() {
       return "DataObject [data1=" + data1 + ", data2=" + data2 + ", list="
    + list + "]";
    }
    public Map<String, Object> getData() {
        return data;
    }

    public void addData(final String key, Object value, Class<T> t) {
        data.put(key, value);
    }
}

public class test {



    /**
 * @param args
 */
public static void main(String[] args) {    
    DataObject obj = new DataObject(12);
    obj.child = new DataObject(25);
    obj.addData("myOtherData", new DataObject(32), DataObject.class);
        Gson gson = new Gson();
        System.out.println(gson.toJson(obj));
    }

}

which is not working:
output:

{"data1":12,"data2":"hello","child":{"data1":25,"data2":"hello","list":["String 1","String 2","String 3"],"data":{}},"list":["String 1","String 2","String 3"],"data":{"myOtherData":{}}}

myOtherData object is missing, because Gson cannot work with general Object

That’s why I started putting the class as third argument of the addData method, I would need to put

Map<String, <T>> data;

I don’t know the way to declare a configurable type in the map

thanks for anyone who will make that work

---

edit : it’s almost working like that

I’m getting the error
Exception in thread “main” java.lang.ClassCastException: DataObject cannot be cast to java.lang.Class
at DataObject.addData(test.java:41)

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.Gson;

class DataObject<T> {

    private int data1 = 100;
    private String data2 = "hello";
DataObject child;
private List<String> list = new ArrayList<String>() {
  {
    add("String 1");
    add("String 2");
    add("String 3");
      }
    };
    private Map<String, Class<T>> data=null;

    public DataObject(int i){
        this.data1 = i;
        this.data = new HashMap<String, Class<T>>();
    }

    //getter and setter methods

    @Override
    public String toString() {
       return "DataObject [data1=" + data1 + ", data2=" + data2 + ", list="
    + list + "]";
    }
    public Map<String, Class<T>> getData() {
        return data;
    }

    public void addData(final String key, Object value, Class<T> t) {
        data.put(key, (Class<T>) value);
    }
}

public class test {



    /**
 * @param args
 */
public static void main(String[] args) {

    DataObject obj = new DataObject(12);
    obj.child = new DataObject(25);
    obj.addData("myOtherData", new DataObject(32), DataObject.class);
        Gson gson = new Gson();
        System.out.println(gson.toJson(obj));
    }

}

Answer 1

EDIT:
Remove the class parameter of the addData() method, it is not needed. Simply type the value with T directly and type datawith Map<String, T>. Initialize the map with: this.data = new HashMap<String, T>();

---

This works for me (I typed your Map value with T and removed the unncesary Class parameter of addData). But your code actually works without those modifications. I ran it on my machine and I had the same output as the one you mark as “working”.

What is a little bit weird, is your typing of DataObject. It is confusing because you have also an inside member (child) which is of the same class. I don’t know if it should be of the same type DataObject<T> or not. You should try to type properly your generics as much as possible. If you are unable to do so, it may be the sign of some incorrect use of generics.

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.Gson;

class DataObject<T> {

    private List<String> list = new ArrayList<String>() {
        {
            add("String 1");
            add("String 2");
            add("String 3");
        }
    };
    private int data1 = 100;
    private String data2 = "hello";
    DataObject child;
    private Map<String, T> data = null;

    public DataObject(int i) {
        this.data1 = i;
        this.data = new HashMap<String, T>();
    }

    // getter and setter methods

    @Override
    public String toString() {
        return "DataObject [data1=" + data1 + ", data2=" + data2 + ", list=" + list + "]";
    }

    public Map<String, T> getData() {
        return data;
    }

    public void addData(final String key, T value) {
        data.put(key, value);
    }
}

public class test {

    /**
     * @param args
     */
    public static void main(String[] args) {
        DataObject obj = new DataObject(12);
        obj.child = new DataObject(25);
        obj.addData("myOtherData", new DataObject(32));
        Gson gson = new Gson();
        System.out.println(gson.toJson(obj));
    }

}