Can't add an object to a Set

Question

Hi I have a little problem in java, I do not have an error but I do not get what I want. Here I create 3 Pizza with and I want to put them in a set, the problem is that when I print my set I only have One Pizza which is the first one p1.

Here's the code :

Pizza p1 = new Pizza(cannibale, Taille.Large, TypePate.Classique);
Pizza p2 = new Pizza(forestiere, Taille.XL, TypePate.Fine);
Pizza p3 = new Pizza(hypnotika, Taille.Medium, TypePate.MozzaCrust);

Set<Pizza> pizzas = new HashSet<Pizza>();

Collections.addAll(pizzas, p1,p2,p3);

Pizza

    @Entity
    @Table(name = "pizza")
    @SequenceGenerator(name = "seqPizza", sequenceName = "seq_pizza", initialValue = 1, allocationSize = 1)

    public class Pizza {
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "seqPizza")
    @Column(name = "id")
private Long id;

@ManyToOne
@JoinColumn(name = "recette", foreignKey = @ForeignKey(name = "PIZZA_RECETTE_ID_FK"))
private Recette recette;

@ManyToOne
@JoinColumn(name = "numticket_id", foreignKey = @ForeignKey(name = "PIZZA_NUMTICKET_ID_FK"))
private Commande commandePizza;

@Column(name = "prix")
private double prix;

@Enumerated(EnumType.STRING)
@Column(name = "taille_pizza")
private Taille taille;

@Enumerated(EnumType.STRING)
@Column(name = "type_pate")
private TypePate pate;

public Pizza() {
}

public Pizza(Recette recette, Taille taille, TypePate pate) {

    this.recette = recette;
    this.taille = taille;
    this.pate = pate;

    if (taille == Taille.Medium) {
        this.prix = recette.getPrixM();
    } else if (taille == Taille.Large) {
        this.prix = recette.getPrixL();
    } else {
        this.prix = recette.getPrixXL();
    }

    if (pate == TypePate.MozzaCrust) {
        this.prix = this.prix + pate.getPrix();
    } else if (pate == TypePate.Pan) {
        this.prix = this.prix + pate.getPrix();
    } else {
        this.prix = this.prix + pate.getPrix();
    }

}

public Long getId() {
    return id;
}

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

public Commande getCommandePizza() {
    return commandePizza;
}

public void setCommandePizza(Commande commandePizza) {
    this.commandePizza = commandePizza;
}

public double getPrix() {
    return prix;
}

public void setPrix(double prix) {
    this.prix = prix;
}

public Recette getRecette() {
    return recette;
}

public void setRecette(Recette recette) {
    this.recette = recette;
}

public Taille getTaille() {
    return taille;
}

public void setTaille(Taille taille) {
    this.taille = taille;
}

public TypePate getPate() {
    return pate;
}

public void setPate(TypePate pate) {
    this.pate = pate;
}

@Override
public int hashCode() {
    return Objects.hash(id);
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Pizza other = (Pizza) obj;
    return Objects.equals(id, other.id);
}

You can find all the code here : https://github.com/HamzaMerini/help/tree/main/pizzayolo

Thank you !!

where is the value of that "id" coming from? You create three instances, all of them with 0 as id, so when you try to add the others, it registers as "already there". re-write your equals method. Don't compare the id's, compare the actual state of the objects — Stultuske, May 27 '22 at 12:31
`HashSet`s use the `hashCode` method to identify and distinguish objects from another. Your `hashCode` only uses the `id` field. You never set the `id` field, meaning it stays `0` and the `HashSet` treats all 3 `Pizza`s as the same thing. Give each pizza a unique id (which is what the database would do) and this will work. — f1sh, May 27 '22 at 12:39
Additional info about hash code overwriting: https://stackoverflow.com/a/2741898/7445342 — SiGe, May 27 '22 at 15:07
Despite of all the obsession with the `hashCode` method shown here, the **`equals`** method is responsible for determining equality. Your equals method says that two objects are equal when they have the same id, so all your three objects are equal. Just check `p1.equals(p2)`, `p2.equals(p3)`, etc. The `hashCode` method has to be compatible to the `equals` method and it is in your code, which is a good thing. Since your objects are mutable, you might stay with this equality and just ensure your objects get unique IDs, as f1sh suggested. — Holger, May 30 '22 at 07:32

score 1 · Answer 1 · answered May 27 '22 at 15:03

Even if your class is annotated with Entity, it won't work similar way when you are instantiating object manually; i.e. while persisting Pizza into db, you might have seen new id got assigned to every insert / save call because of @SequenceGenerator.

But when you do manual instantiation of Pizza & trying to insert it to HashSet, it will check if there is any pre-existing Pizza available based on hashCode. Manual instantiation will assign default value 0 to id & hence only first insertion to HashSet succeed.

Change hashCode so that it will generate some unique value & that way Collections.addAll will work.

score 0 · Answer 2 · answered May 27 '22 at 13:08

HashSets use the hashCode to compute identity. Your pizzas seem to have the same ID. Hence you don't provide one in the constructor nor sets one, it stay null, for all three pizzas. You could persist all those and have the database provide ids, or set them yourself.

When you use at least Java 9, you could use the utility method:


Set<Pizza> pizzas = java.util.Set.of(p1, p2, p3);

For older versions you can use:


Set<Pizza> pizzas = new HashSet<>;
pizzas.add(p1);
pizzas.add(p2);
pizzas.add(p3);

Can't add an object to a Set

2 Answers2