[
  {
    "__typename": "Car",
    "id": "123",
    "name": "Toyota Prius",
    "numDoors": 4
  },
  {
    "__typename": "Boat",
    "id": "4567",
    "name": "U.S.S. Constitution",
    "propulsion": "SAIL"
  }
]

public abstract class Vehicle {
  public final String id;
  public final String name;
}

public class Car extends Vehicle {
  public final Integer numDoors;
}

public class Boat extends Vehicle {
  public final String propulsion;
}

public class SemiParsedKindOfVehicle {
  public final String id;
  public final String name;
  public final Integer numDoors;
  public final String propulsion;
  public final String __typename;
}

import com.squareup.moshi.FromJson;
import com.squareup.moshi.JsonAdapter;
import com.squareup.moshi.Moshi;
import com.squareup.moshi.ToJson;
import com.squareup.moshi.Types;
import org.junit.Assert;
import org.junit.Test;
import java.io.IOException;
import java.lang.reflect.Type;
import java.util.List;
import java.util.Map;
import static org.junit.Assert.assertEquals;

public class Foo {
  static abstract class Vehicle {
    public String id;
    public String name;
  }

  static class Car extends Vehicle {
    public Integer numDoors;
  }

  static class Boat extends Vehicle {
    public String propulsion;
  }

  static class VehicleAdapter {
    @FromJson
    Vehicle fromJson(Map<String, Object> raw) {
      String typename=raw.get("__typename").toString();
      Vehicle result;

      if (typename.equals("Car")) {
        Car car=new Car();

        car.numDoors=((Double)raw.get("numDoors")).intValue();
        result=car;
      }
      else if (typename.equals("Boat")) {
        Boat boat=new Boat();

        boat.propulsion=raw.get("propulsion").toString();
        result=boat;
      }
      else {
        throw new IllegalStateException("Could not identify __typename: "+typename);
      }

      result.id=raw.get("id").toString();
      result.name=raw.get("name").toString();

      return(result);
    }

    @ToJson
    String toJson(Vehicle vehicle) {
      throw new UnsupportedOperationException("Um, why is this required?");
    }
  }

  static final String JSON="[n"+
    "  {n"+
    "    "__typename": "Car",n"+
    "    "id": "123",n"+
    "    "name": "Toyota Prius",n"+
    "    "numDoors": 4n"+
    "  },n"+
    "  {n"+
    "    "__typename": "Boat",n"+
    "    "id": "4567",n"+
    "    "name": "U.S.S. Constitution",n"+
    "    "propulsion": "SAIL"n"+
    "  }n"+
    "]";

  @Test
  public void deserializeGeneric() throws IOException {
    Moshi moshi=new Moshi.Builder().add(new VehicleAdapter()).build();
    Type payloadType=Types.newParameterizedType(List.class, Vehicle.class);
    JsonAdapter<List<Vehicle>> jsonAdapter=moshi.adapter(payloadType);
    List<Vehicle> result=jsonAdapter.fromJson(JSON);

    assertEquals(2, result.size());

    assertEquals(Car.class, result.get(0).getClass());

    Car car=(Car)result.get(0);

    assertEquals("123", car.id);
    assertEquals("Toyota Prius", car.name);
    assertEquals((long)4, (long)car.numDoors);

    assertEquals(Boat.class, result.get(1).getClass());

    Boat boat=(Boat)result.get(1);

    assertEquals("4567", boat.id);
    assertEquals("U.S.S. Constitution", boat.name);
    assertEquals("SAIL", boat.propulsion);
  }
}

private val moshi = Moshi.Builder()
    .add(
      PolymorphicJsonAdapterFactory.of(Vehicle::class.java, "__typename")
        .withSubtype(Car::class.java, "Car")
        .withSubtype(Boat::class.java, "Boat")
    )
    .build()