该方法返回具有子类字段的超类对象

Question

I have a Movie object in the program

public class Movie extends BaseDTO {
      ...
      public static class Builder<T extends Builder> extends BaseDTO.Builder<T> { ... }
}

from this object inherits UserMovie

public class UserMovie extends Movie {
     //there are two additional fields
     private final Integer yourRating;
     private final boolean favorited;

     public static class Builder extends Movie.Builder<Builder> { ... }
}

In some part of the code I want to convert entities to DTO objects. I have two methods. One to convert entities into Movie DTO

static Movie toMovieDto(final MovieEntity movieEntity) {
    return ((Movie.Builder) initMovieDto(movieEntity)).build();
}

the second method to convert to UserMovie DTO

static UserMovie toUserMovieDto(final MovieEntity movieEntity, final UserEntity userEntity) {
    final UserMovie.Builder builder = initMovieDto(movieEntity);

   builder.withYourRating(...);
   builder.withFavorited(userEntity.getFavoritesMovies().contains(movieEntity));

    return builder.build();
}

both methods use a common method to initialize data in the Builder pattern

private static UserMovie.Builder initMovieDto(final MovieEntity movieEntity) {
    final UserMovie.Builder builder = new UserMovie.Builder(
            movieEntity.getTitle(),
            movieEntity.getType()
    )
            .withId(movieEntity.getId().toString());

    builder.withRating(...);
    ...
    return builder;
}

It turns out that the toMovieDto method returns the UserMovie object. Why is this happening? I do not want private fields for UserMovie to be returned in Movie . How to fix it?

Answer 1

Are you worried that someone might cast the Movie returned from toMovieDto into UserMovie ? I wouldn't worry about that much - this requires explicit casting. (Compare it how methods in eg Collections class are defined, or in any Collection implementations).

If you are really worried about security (and only in this case) you need to create another initMovieDto method, that user builder from Movie and not from UserMovie like currently.

So something like this:

private static Movie.Builder initMovieDto(final MovieEntity movieEntity) {
    final Movie.Builder builder = new Movie.Builder(
            movieEntity.getTitle(),
            movieEntity.getType()
    )
            .withId(movieEntity.getId().toString());
    ...
    return builder;
}

static Movie toMovieDto(final MovieEntity movieEntity) {
    return ((Movie.Builder) initMovieDto(movieEntity)).build();
}

and separate for UserMovie:

private static UserMovie.Builder initUserMovieDto(final MovieEntity movieEntity) {
    final UserMovie.Builder builder = new UserMovie.Builder(
            movieEntity.getTitle(),
            movieEntity.getType()
    )
            .withId(movieEntity.getId().toString());

    builder.withRating(...);
    ...
    return builder;
}

static UserMovie toUserMovieDto(final MovieEntity movieEntity, final UserEntity userEntity) {
    final UserMovie.Builder builder = initUserMovieDto(movieEntity);

   builder.withYourRating(...);
   builder.withFavorited(userEntity.getFavoritesMovies().contains(movieEntity));

    return builder.build();
}

This way you will return not only the interface like you want but also concrete class of the same type as the return type in toMovieDto and toUserMovieDto .

Another option would be to define UserMovie builder such that one of it's options would accept a Movie builder.