Question

In my Hibernate classes should instance collections be initialized

public class Basket {
    private List items = new ArrayList();

    ...getters and setters...
}

or left uninitalized

public class Basket {
    private List items;

    ...getters and setters...
}

does it make any kind of difference for Hibernate? I came across this Hibernate documentation where it initializes their HashSet, but I have often seen them left uninitialized.

Answer 1

From Hibernate’s persistent collection documentation:

Due to the underlying relational model, collection-valued properties do not support null value semantics. Hibernate does not distinguish between a null collection reference and an empty collection.

And …

When you make the instance persistent, by calling persist(), Hibernate will actually replace the HashSet with an instance of Hibernate’s own implementation of Set.

These “non-null collection” and “persistent” versus “non-persistent” semantics sometimes gets lost with developers. To keep things simple with Hibernate objects, I prefer:

• always initialize all Collections with java.util implementations
• always code to Collection interfaces

Making it customary for Hibernate object Collections never being NULL and avoiding the pitfall noted in the above documentation of casting a Hibernate object Collection to an invalid implementation.