Serialize and deserialize nested classes

Question

I'm using Gson in order to save my classes in a backwards compatible way. I have the following singleton class which I'm trying to serialize.

public class DataHandler implements Serializable {
      private static DataHandler instance; //SINGLETON
      private HashMap<FormIdentification, BaseForm> forms =
        new HashMap<FormIdentification, BaseForm>();

      public void read(){
        //read the json string
        Gson gson = new Gson();
        instance = gson.fromJson(json, DataHandler.class);      
      }

      public void write(){
        Gson gson = new Gson();
        String json = gson.toJson(instance);
        //write the json string
      }
}

public class FormIdentification implements Serializable{
    public String name, type;
    byte[] id;

    public FormIdentification(String name, String type, byte[] id) {
        this.name = name;
        this.type = type;
        this.id = id;
    }
}

public abstract class BaseForm implements Serializable{
    protected FormIdentification identity;
    protected final List<BaseQuestion> questions = new ArrayList<BaseQuestion>();
    protected final HashMap<String, List<String>> personalData = new HashMap<String, List<String>>();
}

public class LinearForm extends BaseForm {
    private DefaultMutableTreeNode criteria;
}

public abstract class BaseQuestion implements Serializable{
    protected String question;
    protected List<String> possibleAnswers;
}

It serializes to the following json:

{
       "forms":{
          "test.Structures.FormIdentification@7b875faa":{
             "criteria":{
                "allowsChildren":true
             },
             "identity":{
                "name":"Mircea",
                "type":"linear",
                "id":[
                   111,
                   119
                ]
             },
             "questions":[

             ],
             "personalData":{
                "Nume":[

                ]
             }
          }
       }
    }

The json is complete and has all the data that the classes contained when it was generated. But when I'm trying to deserialize it gives the following error:

SEVERE: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 1 column 12

Note that it was compact formatted, so line 1 column 12 should be: {"forms":{"test.Structures..."

I'm new to json, so I started searching for a solution, but I haven't found one. I tried casting using a TypeToken and I've looked into writing a custom serializer, but haven't managed that. I also tried serializing only the "forms" variables but it gives the same error.

You should put your code in question. Don't post it as links which can die in some time making your question impossible to understand. — Pshemo, Jan 25 '14 at 16:48
@Pshemo Thank you for your suggestion. I've done as you said. — mmimaa, Jan 25 '14 at 16:55

score 1 · Accepted Answer · answered Jan 25 '14 at 19:16

1

A JSON object is a collection of key/value pairs. The values can be any JSON value: string, number, object, array, true, false, or null. However, the key in a JSON object can only be a string.

When JSON-serializing Map<FormIdentification, BaseForm>, your key is FormIdentification. The best GSON can do is use FormIdentification.toString() to get the string for the key.

However, during JSON-deserialization there is no way to know how to get a FormIdentification from a string.

answered Jan 25 '14 at 19:16

Glenn Lane

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Thank you for your answer. Now that you said it, when I look at the json it's clear to me why it's not working. I switched to XML using xStream in order to achieve the same thing. – mmimaa Jan 25 '14 at 23:10
You could consider a custom JSON serialization, where your map is expressed as a JSON array of objects, where the object has keys `key` and `value` to express each `Map` entry pair: [{"key": {`the serialized key1`}, "value": {`the serialized value1`}}, {"key": {`the serialized key2`}, "value": {`the serialized value2`}}, ...] – Glenn Lane Jan 26 '14 at 21:10

Serialize and deserialize nested classes

1 Answers1