Mapping.java

interface EntityVisitor<T> {
    T visitGood(Good good);
    T visitBad(Bad bad);
}

class Good {
    <T> T accept(EntityVisitor<T> visitor) {
        return visitor.visitGood(this);
    }
}

class Bad {
    <T> T accept(EntityVisitor<T> visitor) {
        return visitor.visitBad(this);
    }
}

public class UberMapper implements EntityVisitor<XXXDTO> {
    private GoodMapper goodMapper;
    private BadMapper badMapper;

    @Autowired
    public UberMapper(GoodMapper goodMapper, BadMapper badMapper) {
        this.goodMapper = goodMapper;
        this.badMapper = badMapper;
    }

    @Override
    public XXXDTO visitGood(Good good) {
        return goodMapper.map(good);
    }

    @Override
    public XXXDTO visitBad(Bad bad) {
        return badMapper.map(bad);
    }
}

public class XXXService {

    private final UberMapper uberMapper;

    @Autowired
    public XXXService(UberMapper uberMapper) {
        this.uberMapper = uberMapper;
    }

    public Stream<XXXDTO> aggregate(Stream<Bad> badStream, Stream<Good> goodStream) {
        return Stream.concat(badStream.map(bad -> bad.accept(uberMapper)),
                             goodStream.map(good -> good.accept(uberMapper)));
    }
}