Hibernate version: 3.1.2

I'm trying to make an association that is optional, i.e. class A may reference an instance of B, but the reference may also be null.

The assocation is declared as:



But this is causing a NullPointerException when fetching the object having this association.

Is there some way of declaring [many|one]-to-one-or-zero that I don't know about? The only other solution I can imagine is use a collection association as the collection can be empty, but that's not a very satisfactory solution.

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Hibernate will not create any rows for an empty collection, and for a relational database, this is the same as many-to-one-or-none. This tricky bit is that hibernate will call setCollection(null) when loading that empty collection, and this causes a null pointer exception when an internal hash code is being calculated. Put code in you getCollection (or setCollection) method that ensures that your collection object is never null:If you're using field access, you'll have to put this in the set method. For property access (the default) you can put it in either.