How to represent a relationship N: N in java?

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How to represent a relationship N: N in java?

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#1 by (2 votes)

1

I think it's a simple question, but I could not find any examples that would make that clear. I have already done my database model in PostGreSQL and at the time of creating the classes I got a little lost. I have List entities (stores user's favorite movies) and Movie (which stores movie data) and there is a N: N relationship between these entities, ie multiple movies can be associated with multiple lists and vice versa. I created a ListaXFilme table that will have 2 foreign keys (one to identify the other one to identify the list) and the primary key of this table will be composed by the 2 foreign keys. I think I've been doing it right ... My question is how do I create classes that represent this relationship.

The table scripts I mentioned:

CREATE TABLE app.tb_lista_favoritos(
id SERIAL NOT NULL,
nome VARCHAR(45) NOT NULL,
data_criacao DATE NOT NULL,
CONSTRAINT pk_lista_favoritos PRIMARY KEY (id)
);

CREATE TABLE app.tb_filme(
cod SERIAL NOT NULL,
nome VARCHAR(45) NOT NULL,
data_lancamento DATE NOT NULL,
genero VARCHAR(45) NOT NULL,
duracao TIME NOT NULL,
avaliacao REAL NOT NULL,
CONSTRAINT pk_filme PRIMARY KEY (cod)
);

CREATE TABLE app.tb_listaXfilme(
id_lista INT NOT NULL,
cod_filme INT NOT NULL,
CONSTRAINT pk_listaXfilme PRIMARY KEY (id_lista,cod_filme),
CONSTRAINT fk1_listaXfilme FOREIGN KEY (id_lista) REFERENCES app.tb_lista_favoritos (id),
CONSTRAINT fk2_listaXfilme FOREIGN KEY (cod_filme) REFERENCES app.tb_filme (cod)
);

Classes:

Favorites.java:

public class ListaFavoritos {
    int codLista;
    String nomeLista;
    Date dataCriacao;

    public ListaFavoritos(int codLista, String nomeLista, Date dataCriacao) {
        this.codLista = codLista;
        this.nomeLista = nomeLista;
        this.dataCriacao = dataCriacao;
    }

    public ListaFavoritos() {

    }

    /*Getters e Setters*/
}

Movie.java:

public class Filme {
    int codFilme;
    double avaliacao;
    Date anoLancamento,duracao;
    String nome, genero;

    public Filme(int codFilme, double avaliacao, Date anoLancamento, Date duracao, String nome, String genero) {
        this.codFilme = codFilme;
        this.avaliacao = avaliacao;
        this.anoLancamento = anoLancamento;
        this.duracao = duracao;
        this.nome = nome;
        this.genero = genero;
    }

    public Filme() {
    }

    /*Getters e setters*/

}

java classes modelagem

asked by anonymous 16.10.2016 / 01:51

1 answer

Retrieve callback return Function rand () does not return random numbers. C ++

score 2 · Accepted Answer

In database, relationship n to n generates another intermediate table to store the ids of both sides.

When we think of objects, it gets a bit different.

In this case it makes sense in the FavoritesList class to have an attribute that stores a list of Movies, thus:

Movie.java:

public class Filme {
    int codFilme;
    double avaliacao;
    Date anoLancamento,duracao;
    String nome, genero;

    public Filme(int codFilme, double avaliacao, Date anoLancamento, Date duracao, String nome, String genero) {
        this.codFilme = codFilme;
        this.avaliacao = avaliacao;
        this.anoLancamento = anoLancamento;
        this.duracao = duracao;
        this.nome = nome;
        this.genero = genero;
    }

    public Filme() {
    }

    /*Getters e setters*/

}

FavoriteFlags.java

public class ListaFavoritos {
    int codLista;
    String nomeLista;
    Date dataCriacao;
    List<Filme> filmes;

    public ListaFavoritos(int codLista, String nomeLista, Date dataCriacao, List<Filme> filmes) {
        this.codLista = codLista;
        this.nomeLista = nomeLista;
        this.dataCriacao = dataCriacao;
        this.filmes = filmes;
    }

    public ListaFavoritos() {

    }

    /*Getters e Setters*/
}