JSONObject.toString() is creating OutOfMemoryError

I have to convert StackExchange's dataset to Json file, so I have tried this-

public class Parser {
    public static int PRETTY_FACTOR=4;
    public static void main(String[] args) throws Exception {  
        String fileName = "/home/dipannoy/Desktop/stackexchange/android/posthistory.json";
        String path= "/home/dipannoy/Desktop/stackexchange/android/PostHistory.xml";

        try {           

            StringBuilder builder =  new StringBuilder();  


            FileInputStream inputStream = null;
            Scanner sc = null;
            try {
                inputStream = new FileInputStream(path);
                sc = new Scanner(inputStream, "UTF-8");
                while (sc.hasNextLine()) {
                    String line = sc.nextLine();
                    builder.append(line);
                }
                if (sc.ioException() != null) {
                    throw sc.ioException();
                }
            } finally {
                if (inputStream != null) {
                    inputStream.close();
                }
                if (sc != null) {
                    sc.close();
                }
            }


            String xml  = builder.toString();  
            JSONObject jsonObj = XML.toJSONObject(xml);   


            FileWriter fileWriter = new FileWriter(fileName);
            BufferedWriter bufferedWriter =new BufferedWriter(fileWriter);

            bufferedWriter.write(jsonObj.toString());
            bufferedWriter.flush();            
            bufferedWriter.close();
            fileWriter.close();
        }


          catch(IOException ex) {
                System.out.println(
                    "Error writing to file '"
                    + fileName + "'");

            } catch(Exception e) {  
                e.printStackTrace();  
            }
    }  
}

But it was having error at jsonObj.toString() . A sample xml is -

<comments>
<row Id="1" PostId="1" Score="4" Text="Did I just place the first upvote?  Congrats on getting this site off the ground!" CreationDate="2016-01-12T18:47:12.573" UserId="23"/>
</comments>

I have tried to use Gson but failed to convert GSONObject to GsonObject as GsonParser need to have GSONObject's toString() method which is creating the OutOfMemoryError. Can anyone help in this matter?