How can we handle different response type with Retrofit 2?

I had a similar issue and I solved it by using a generic Object and then testing what type of response I had using instanceof

Call<Object> call = api.login(username, password);
call.enqueue(new Callback<Object>() 
{
    @Override
    public void onResponse(Response<Object> response, Retrofit retrofit)
    {
         if (response.body() instanceof MyPOJO )
         {
            MyPOJO myObj = (MyPOJO) response.body();
            //handle MyPOJO 
         }
         else  //must be error object
         {
            MyError myError = (MyError) response.body();
            //handle error object
         }
    }

    @Override
    public void onFailure(Throwable t) 
    {
     ///Handle failure
    }
});

In my case I had either MyPOJO or MyError returned and I could be sure it would be one of these.

In other cases then I had the backend return the same Response Object no matter if the request was successful or not.
Then inside this response object I had my actual data within an “Object” field. Then I can use instance of to determine what type of data I had. In this case I always had the same object being returned, no matter what the call was.

public class MyResponse {

    private int responseCode;
    private String command;
    private int errorno;
    private String errorMessage;
    private Object responseObject;   //This object varies depending on what command was called
    private Date responseTime;
}