First class support for polymorphism (subclasses!)

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GSON always operates on the static type of an object. It has no mechanism to 
operate on the object's runtime type.

It would be handy if GsonBuilder permitted a way to specify the known 
subclasses of a type. Then at serialization/deserialization these fields could 
be included:

  public void testPolymorphism() {
    Rectangle r = new Rectangle();
    r.width = 5;
    r.height = 7;
    Circle c = new Circle();
    c.radius = 3;

    List<Shape> shapes = new ArrayList<Shape>();
    shapes.add(r);
    shapes.add(c);

    Gson gson = new GsonBuilder()
        .create();

    String json = gson.toJson(shapes, new TypeToken<List<Shape>>() {}.getType());
    assertEquals("[{\"width\":5,\"height\":7},{\"radius\":3}]", json);
  }

  static class Shape {}

  static class Rectangle extends Shape {
    int width;
    int height;
  }

  static class Circle extends Shape {
    int radius;
  }

It would be extra awesome if it could use the set of fields in a stream to 
infer the type to instantiate. In the example above, it could use that the 
'radius' field as evidence that the runtime type should be a Circle. Or perhaps 
this could be user-configured too, such as this:
  new GsonBuilder()
    .runtimeType(Shape.class, Circle.class, "radius")
    .runtimeType(Shape.class, Rectangle.class, "width", "height")
    .create();

Original issue reported on code.google.com by limpbizkit on 28 Aug 2010 at 6:00