GSON converts to LinkedHashMap instead of my object

Question

I have this snippet of code:

public abstract class Repository<Entity extends BaseObject> {

...

public void readFromJson(){
    String content = "JSON content here";
    Gson gson = new Gson();
    Type entityType = new TypeToken<JSONObject<Entity>>(){}.getType();

    jsonObject = gson.fromJson(content, entityType);

    for (Entity ent : jsonObject.getEntities()) ;
    }
}

When I try to do the foreach my entities object is no longer of type Entity but LinkedHashMap and I get this exception: java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.tranca.bookstore.domain.shared.BaseObject

Here is the JSONObject class(created by me)

public class JSONObject<Entity> {

private List<Entity> entities = new ArrayList<Entity>();
private long lastId = -1;
public List<Entity> getEntities() {
    return entities;
}
public void setEntities(List<Entity> entities) {
    this.entities = entities;
}
public long getLastId() {
    return lastId;
}
public void setLastId(long lastId) {
    this.lastId = lastId;
}

public void incrementLastId() {
    this.lastId++;
}

}

maybe the base object is relevant so I will put the code here:

public abstract class BaseObject implements Serializable {

protected long id = (long) -1;
protected int version = 0;

protected BaseObject(){}

public long getId() {
    return id;
}
public void setId(long id) {
    this.id = id;
}
public int getVersion() {
    return version;
}
public void setVersion(int version) {
    this.version = version;
}

}

Answer 1

I had the same / a similar problem. To give a more clear answer in a slightly different context:

I had following Method which produced the error "com.google.gson.internal.LinkedTreeMap cannot be cast to MyType":

/**
   * Reads a LinkedHashMap from the specified parcel.
   * 
   * @param <TKey>
   *          The type of the key.
   * @param <TValue>
   *          The type of the value.
   * @param in
   *          The in parcel.
   * @return Returns an instance of linked hash map or null.
   */
  public static <TKey, TValue> LinkedHashMap<TKey, TValue> readLinkedHashMap(Parcel in) {
    Gson gson = JsonHelper.getGsonInstance();
    String content = in.readString();
    LinkedHashMap<TKey, TValue> result = gson.fromJson(content, new TypeToken<LinkedHashMap<TKey, TValue>>(){}.getType());
    return result;
  }

I wanted an easy generic way to read/write linked hashmap. The above solution does not work because the type information of the TypeToken with TKey an TValue will be lost after compilation as far as i understand. And this is the problem. If you change you're code to following example, then it works, because now we explicitly define the type token. I am not so much into java that i understand why in this case it is possible to read the type information at runtime.

/**
   * Reads a LinkedHashMap from the specified parcel.
   * 
   * @param <TKey>
   *          The type of the key.
   * @param <TValue>
   *          The type of the value.
   * @param in
   *          The in parcel.
   * @return Returns an instance of linked hash map or null.
   */
  public static <TKey, TValue> LinkedHashMap<TKey, TValue> readLinkedHashMap(Parcel in, TypeToken<LinkedHashMap<TKey, TValue>> typeToken) {
    Gson gson = JsonHelper.getGsonInstance();
    Type type = typeToken.getType();
    String content = in.readString();
    LinkedHashMap<TKey, TValue> result = gson.fromJson(content, type);
    return result;
  }

And now you would call the above function like:

readLinkedHashMap(in, new TypeToken<LinkedHashMap<UUID, MyObject>>(){});

A sidenote 1: When writign the linked hash map, you do not need to specify any type token at all. toJson(map) is sufficient.

A sidenote 2 (to a problem which I had): By default gson uses toString() to serialize the key. If you register a type adapter for the key type which is maybe a more complex type, then this type adapter is not applied when serializing, but when deserializing. This leads to a non consistent and therefore failing process. Following options activates complex map key serialization.

gsonBuilder.enableComplexMapKeySerialization()

Answer 2

Finally got it!

The problem was that:

new TypeToken< JSONObject< Entity>>(){}.getType();

returns the type of JSONObject< T> not the specific entity of the subclass that was extending Repository(eg UserRepository extends Repository< User>).

The trick was to create an abstract method to force the subclasses to set the Type for deserialization.

In conclusion if you get this error be sure you have the right type of class (in case you use subclasses be sure it returns the type of you subclass not superclass).