Gson.toJson(Object) returning object hash code

Question

I'm trying to transform this class below using the Gson.ToJson(Object) method, but it is returing me the object hash code of the class, eg: br.com.helpradar.entity.User@475fdaaa

However, I can retrieve the user.expertise without any problems and with all the relationships: Gson.ToJson(user.getExpertise)

@Entity
@SequenceGenerator(name="seqUser", sequenceName="SEQ_USER", allocationSize=1)
public class User {
@Id
private Long id;

@Column(nullable=false)
private String name;

@OneToOne
private Contact contact;

//enum
private TypeUser typeUser;

@ManyToMany(cascade = { CascadeType.ALL })  
@JoinTable(name = "USER_REVIEW", 
joinColumns = { @JoinColumn(name = "USER_ID") },
inverseJoinColumns = { @JoinColumn(name = "REVIEW_ID") })  
@Column(name="REVIEW")
private Set<Review> review= new HashSet<Review>();

@ManyToMany(cascade = { CascadeType.ALL })  
@JoinTable(name = "USER_EXPERTISE", 
joinColumns = { @JoinColumn(name = "USER_ID") },
inverseJoinColumns = { @JoinColumn(name = "EXPERTISE_ID") })  
@Column(name="EXPERTISE")
private Set<Expertise> expertise = new HashSet<Expertise>();
}

This is my Gson method:

Gson gson = new GsonBuilder()
    .registerTypeAdapter(User.class, new MyTypeAdapter<Expertise>())
    .registerTypeAdapter(User.class, new MyTypeAdapter<Review>())

    .create();

return gson.toJson(user);

This is my MyTypeAdapter:

class MyTypeAdapter<T> extends TypeAdapter<T> {
public T read(JsonReader reader) throws IOException {
    return null;
}

public void write(JsonWriter writer, T obj) throws IOException {
    if (obj == null) {
        writer.nullValue();
        return;
    }
    writer.value(obj.toString());
}

}

So, how do I get the Gson.ToJson(user) to actually return a Json string so that I can use Gson.FromJson on my other end? Thanks in advance.

Answer 1

I think you need use method enableComplexMapKeySerialization() . Here you can see next documentation:

public GsonBuilder enableComplexMapKeySerialization()

Enabling this feature will only change the serialized form if the map 
key is a complex type (i.e. non-primitive) in its serialized JSON form. 
The default implementation of map serialization uses toString() on the key...

And example:

Gson gson = new GsonBuilder()
   .register(Point.class, new MyPointTypeAdapter())
   .enableComplexMapKeySerialization()
   .create();

Map<Point, String> original = new LinkedHashMap<Point, String>();
original.put(new Point(5, 6), "a");
original.put(new Point(8, 8), "b");
System.out.println(gson.toJson(original, type));

Output will be:

{
     "(5,6)": "a",
     "(8,8)": "b"
}

So, you can try like this:

Gson gson = new GsonBuilder()
.registerTypeAdapter(User.class, new MyTypeAdapter<Expertise>())
.registerTypeAdapter(User.class, new MyTypeAdapter<Review>())
.enableComplexMapKeySerialization()
.create();