How to avoid duplicate code due to the impossibility of using use multiple inheritance. Hibernate

Question

I'm working on a Spring - Hibernate App, and I have a question about how to correctly avoid duplicate code and using Hibernate, due to the impossibility of using use multiple inheritance (I usually work with Python so this is not a "problem").

My UML:

https://i.stack.imgur.com/LdEwf.png

My class, Periodico, for example, the same for Livro or Prototexto extends GenericEntity:

@Entity
@Table(name = "periodico")
public class Periodico extends GenericEntity {

}

My question is: ¿what is the way to implement inheritance in this case of two classes?

At this moment I'm doing this and it works. But I duplicate some code (Produçao activa) in each class.

/**
 * Created by hlorenzo on 03/08/2017.
 */
@Entity
@Table(name = "prototexto")
public class Prototexto extends GenericEntity {


    private String titulo;

    /*
     Código alfanumérico composto por nº de clase + tipo (3 iniciais) + nº Id (3 cifras) + data_ano
    */
    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "numerasao")
    private String numerasao;

    /*
     La utilización del prefijo nacimiento es para que funcione con el componente fecha.component.js de forma automática.
     */
    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "publicacion_dia")
    private Integer nacimientoDia;

    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "publicacion_mes")
    private Integer nacimientoMes;

    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "publicacion_ano")
    private Integer nacimientoAno;

    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "primeira_linha")
    private String primeiraLinha;

    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "ultima_linha")
    private String ultimaLinha;

    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "numero_paginas")
    private Long numeroPaginas;

    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "descrisao")
    private String descrisao;

    @JsonView(JsonViews.DetailedList.class)
    @Column(name = "localizasao")
    private String localizasao;

    @Enumerated(value = EnumType.STRING)
    private TipoPrototexto tipo;


    private boolean revisado;

    private boolean concluido;

    @JsonView(JsonViews.DetailedList.class)
    @ManyToOne
    @JoinColumn(name="pais_id")
    private Pais pais;


    @JsonView(JsonViews.DetailedList.class)
    @ManyToOne
    @JoinColumn(name="ciudad_id")
    private Ciudad ciudad;
[...]
}

And my GenericEntity:

@MappedSuperclass
public abstract class GenericEntity implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @JsonView(JsonViews.List.class)
    private Long id;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

}

Thank you so much.

Answer 1

In many cases where you could use inheritance from multiple parent classes, you should instead use either interfaces (if you need to write your own implementation in the subclass, and just want to declare that you are compatible with the interfaces) or use aggregation (if you don't need to overwrite anything, just want to re-use some code, aggregation is often much more flexible).

So instead of extending GenericEntity AND Publicada, you could extend GenericEntity and aggregate Publicada.

If storing this into a database, a ORM mapper does anyway "convert" inheritance into aggregations, so its anyway much easier for your brain if you use aggregation in the first place instead of letting hibernate convert your inheritance into aggregation so that it can be stored into a database.

If you read about design pattern (there are many books about it, like "heads up design pattern" first chapter https://www.google.com/?q=head+first+design+patterns+pdf -> page 13 or google for aggregation over inheritance Inheritance vs. Aggregation or https://en.wikipedia.org/wiki/Composition_over_inheritance), you can see why aggregation has often many advantages over inheritance and why they decided to disallow inheritance from multiple classes in java.