Populate existing object

It would be great is gson supported the population of existing objects. There 
was a post a while back alluding to this behavior, but it doesn't exist now:

http://groups.google.com/group/google-gson/browse_thread/thread/ab97b754c1f3c53a
?pli=1

The rationale is that this would prevent users from having to create 
TypeAdapters or InstanceCreators for every object that requires custom 
construction, particularly since gson doesn't support the notion of a global 
InstanceCreator at the moment.

Original issue reported on code.google.com by [e-mail] on 8 Apr 2012 at 5:12

[mbalick]

I’ve been looking for this functionality for years. @inder123 said “We are still searching for an elegant proposal.” What’s wrong with something like gson.fromJson(existingObject, json) ? I don’t see why it would not cover the same use cases as gson.fromJson(json).

[inder123]

You can implement this using InstanceCreators.

final Foo existing;
InstanceCreator<Foo> creator = new InstanceCreator<Foo>() {
  public Foo createInstance(Type type) { return existing; }
}

Gson gson = new GsonBuilder().registerTypeAdapter(Foo.class, creator).create();
Foo value = gson.fromJson(jsonString, Foo.class);
// value should be same as existing