Question

I have an object that contains

public class PositionsChannelApplicationGroups {

public PositionsChannelApplicationGroups(){}

    private Map<MyObj1, List<Character>> portfoliosToApplicationIds = new HashMap<MyObj1, List<Character>>();
    private Map<MyObj1, List<Character>> accountsToApplicationIds = new HashMap<MyObj2, List<Character>>();

    private Map<Character, List<MyObj1>> applicationIdToPortfolios = new HashMap<Character, List<MyObj1>>();
    private Map<Character, List<MyObj2>> applicationIdToAccounts = new HashMap<Character, List<MyObj2>>();
 }

Now I try to gson it at the server and de-gson it at the client.
To make it simple I get an exception when I do this in one line

 Gson gson = new Gson();
 gson.fromJson(gson.toJson(object), PositionsChannelApplicationGroups.class);

or even

 gson.fromJson(gson.toJson(object), new TypeToken<PositionsChannelApplicationGroups>(){}.getType());

but it gives me the following exception (below ).
what am I doing wrong ?

com.google.gson.JsonParseException: Expecting object found: "MyObj1{hibernateID=0, portfolioName='MyString'}"
    at com.google.gson.JsonObjectDeserializationVisitor.visitFieldUsingCustomHandler(JsonObjectDeserializationVisitor.java:100)
    at com.google.gson.ReflectingFieldNavigator.visitFieldsReflectively(ReflectingFieldNavigator.java:63)
    at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:120)
    at com.google.gson.JsonDeserializationContextDefault.fromJsonPrimitive(JsonDeserializationContextDefault.java:85)
    at com.google.gson.JsonDeserializationContextDefault.deserialize(JsonDeserializationContextDefault.java:56)
    at com.google.gson.MapTypeAdapter.deserialize(MapTypeAdapter.java:67)
    at com.google.gson.MapTypeAdapter.deserialize(MapTypeAdapter.java:33)
    at com.google.gson.JsonDeserializerExceptionWrapper.deserialize(JsonDeserializerExceptionWrapper.java:51)
    at com.google.gson.JsonDeserializationVisitor.invokeCustomDeserializer(JsonDeserializationVisitor.java:92)
    at com.google.gson.JsonObjectDeserializationVisitor.visitFieldUsingCustomHandler(JsonObjectDeserializationVisitor.java:117)
    at com.google.gson.ReflectingFieldNavigator.visitFieldsReflectively(ReflectingFieldNavigator.java:63)
    at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:120)

Answer 1

The Gson limitation you’re bumping up against concerns how it serializes map keys: by calling toString(). From MapTypeAdapter:

map.add(String.valueOf(entry.getKey()), valueElement);

This behavior is also described in the MapTypeAdapter documentation.

This implementation really only works well with simple primitive types as the map key. If the key is not a simple primitive then the object is {@code toString}ed and that value is used as its key.

If you insist on using custom types as map keys, then as best I can tell you’re going to have to write a custom serializer and/or a custom deserializer and/or a toString() that generates a string representation that’s easy to deserialize.

Also, take a look at MapAsArrayTypeAdapter for one approach. (It’s usable with a call to GsonBuilder.enableComplexMapKeySerialization(), not through direct instantiation as the docs describe (because it’s currently not a public class). I didn’t test it to see if its implementation works, but it looks promising.)

Note: The applicationIdToPortfolios and applicationIdToAccounts attributes serialize and deserialize simply without custom handling, since they are maps with primitive type keys.