GWT JsInterop DTOs inheritance

GeoJson.java

import static com.fasterxml.jackson.annotation.JsonTypeInfo.As.EXISTING_PROPERTY;
import static com.fasterxml.jackson.annotation.JsonTypeInfo.Id.NAME;
import static jsinterop.annotations.JsPackage.GLOBAL;

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonTypeName;
import javax.annotation.Nullable;
import jsinterop.annotations.JsOverlay;
import jsinterop.annotations.JsType;

@JsonTypeInfo(use = NAME, include = EXISTING_PROPERTY, property = "type", visible = true)
@JsonSubTypes({
        @JsonSubTypes.Type(value = GeoJson.Feature.class, name = "Feature"),
        @JsonSubTypes.Type(value = GeoJson.FeatureCollection.class, name = "FeatureCollection")
})
@JsType(namespace = GLOBAL, name = "Object", isNative = true)
class GeoJson {
    public String type;
    public final @JsOverlay @JsonIgnore
    Type getTypeEnum() { return Type.valueOf(type); }
    public final @JsOverlay @JsonIgnore void setTypeEnum(Type type) { this.type = type.name(); }

    public static @JsOverlay FeatureCollection featureCollection(Feature... features) {
        FeatureCollection o = new FeatureCollection();
        o.setTypeEnum(Type.FeatureCollection);
        o.features = features;
        return o;
    }

    public static @JsOverlay Feature feature(Geometry geometry) { return feature(null, geometry); }
    public static @JsOverlay Feature feature(@Nullable String featureId, Geometry geometry) {
        Feature o = new Feature();
        o.setTypeEnum(Type.Feature);
        o.id = featureId;
        o.geometry = geometry;
        return o;
    }

    public static @JsOverlay Point point(double x, double y) { return point(new double[] { x, y }); }
    public static @JsOverlay Point point(double[] coordinates) {
        Point o = new Point();
        o.setTypeEnum(Geometry.Type.Point);
        o.coordinates = coordinates;
        return o;
    }

    public static @JsOverlay Polygon polygon(double[][] coordinates) {
        Polygon o = new Polygon();
        o.setTypeEnum(Geometry.Type.Polygon);
        o.coordinates = new double[][][] { coordinates };
        return o;
    }

    public enum Type {Feature, FeatureCollection}

    @JsType(namespace = GLOBAL, name = "Object", isNative = true)
    public static final class Feature extends GeoJson {
        public @Nullable String id;
        public Geometry geometry;
    }

    @JsType(namespace = GLOBAL, name = "Object", isNative = true)
    public static class FeatureCollection extends GeoJson {
        public Feature[] features;
    }

    @JsonTypeInfo(use = NAME, include = EXISTING_PROPERTY, property = "type", visible = true)
    @JsonSubTypes({
            @JsonSubTypes.Type(value = Point.class, name = "Point"),
            @JsonSubTypes.Type(value = Polygon.class, name = "Polygon")
    })
    @JsType(namespace = GLOBAL, name = "Object", isNative = true)
    public static abstract class Geometry {
        public String type;
        public final @JsOverlay @JsonIgnore Geometry.Type getTypeEnum() { return Geometry.Type.valueOf(type); }
        public final @JsOverlay @JsonIgnore void setTypeEnum(Geometry.Type type) { this.type = type.name(); }

        public final @JsOverlay <T> T accept(GeometryVisitor<T> fn) { switch (getTypeEnum()) {
            case Point: return fn.point((Point) this);
            case Polygon: return fn.polygon((Polygon) this);
            default: throw new UnsupportedOperationException("unexpected type " + type);
        } }

        public static @JsOverlay @Nullable Point isPoint(@Nullable Geometry g) {
            return g == null ? null : g.accept(new GeometryVisitor<Point>() {
                @Override public Point point(Point g) { return g; }
                @Override public Point polygon(Polygon p) { return null; }
            });
        }

        public static @JsOverlay @Nullable Polygon isPolygon(@Nullable Geometry g) {
            return g == null ? null : g.accept(new GeometryVisitor<Polygon>() {
                @Override public Polygon point(Point g) { return null; }
                @Override public Polygon polygon(Polygon p) { return p; }
            });
        }

        public enum Type {Point, Polygon}
    }

    @JsonTypeName("Point") @JsType(namespace = GLOBAL, name = "Object", isNative = true)
    public static class Point extends Geometry {
        public double[] coordinates;
        public final @JsOverlay @JsonIgnore double x() { return coordinates[0]; }
        public final @JsOverlay @JsonIgnore double y() { return coordinates[1]; }
    }

    @JsonTypeName("Polygon") @JsType(namespace = GLOBAL, name = "Object", isNative = true)
    public static final class Polygon extends Geometry {
        public double[][][] coordinates;
        public final @JsOverlay @JsonIgnore double[][] shell() { return coordinates[0]; }
    }

    public interface GeometryVisitor<T> {
        T point(Point g);
        T polygon(Polygon p);
    }
}