RE: [Springframework-developer] equals/hashCode for entities

[<jue...@we...>]

<quote>
So, we make a hashCode with some smarts. First time hashcode is called,
- if id is null/zero, then hashCode will forever return the same value,=20
which is the same for all class instances. Inefficient, but doesn't=20
cause Sets to barf.
- but if is not-null/zero and hashCode has never been called while id=20
was null/zero, then hashCode will retun a value based on the id. This=20
means that loaded entities loaded form the db are treated efficiently.

Still not that great in terms of efficiency for new entities, but=20
efficient for old entities.
</quote>

Interesting idea - a hashCode implementation that tracks if it has =
already been called with an empty ID... So "contains" and "remove" will =
work even after saving, but unfortunately only for this very instance. =
Calling "contains" with a freshly loaded instance (and the previously =
saved one in the collection) will still fail, as the freshly loaded =
instance will return an ID-based hash code that will not match the one =
in the collection.

<quote>
Also, there is still an issue with any collection (ie not HashSet) that=20
uses 'equals', since that is going to change when the id changes.
</quote>

But that doesn't matter as such a collection will just call "equals" on =
*lookup*, not on *addition*. It will always find an entity, as long as =
"equals" can deal with the current state. That's not the case with =
HashSet: The hash code of an object is determined within the "add" =
method, and fixed from there on.

Juergen