Should JPA entities return a constant for the hashcode() implementation?

Question

This article suggests a JPA

equals

and

hashcode

implementation that returns

31

always for the

hashcode

. Would this not become a problem when we try to add entitities to a HashMap or HashSet?

In other words if I try insert a two JPA entity instances of type Product like this:

//Load product1 and product2

Set<Product> products = new HashSet<>();

products.add(product1);

products.add(product2);

Will the products

Set

now contain two products even though the

hashcode

for both of these products is the same?

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Answer 1

Definitely. See Class HashMap documentation. HashSet is backed by a HashMap, so this applies to HashSet too. See Class HashSet documentation.

To the second question, no. HashMap uses equality to decide whether an entity is equal. It will just be terribly inefficient since the map will degenerate to a linked list.

Should JPA entities return a constant for the hashcode() implementation?

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