[Pojomatic-users] Handling annotations on interfaces

[Ian R. <ian...@gm...>]

Currently, Pojomatic ignores annotations on any interfaces a class might
implement.  Moreover, it is not possible to create a pojomator for an
interface, since ClassProperties walks up the superclass hierarchy until it
hits Object.class, but  the superclass of an interface is null, so we get a
NullPointerException.

I can see at least two use cases for supporting Pojomators for interfaces:

   1. If an interface defines the behavior of equals and  hashCode as part
   of its contract, it could include @Property annotations and a public static
   Pojomator instance which implementing classes could use for their equals and
   hashCode implementations.
   2. If someone needs to be able to check equality on a subset of
   properties of SomeClass (independently of SomeClass's equals method), they
   can define an interface with only those properties, annotate those
   properties (on the interface) and then have SomeClass implement that
   interface.  Finally, they could use the pojomator for the interface to check
   equality of just the properties in question.

What I would like to propose is that if ClassProperties is constructed for
an interface, that it walk up the implementors hierarchy instead of the
superclass hierarchy; classes would continue to be handled as they are
today.  As such, this is a non-breaking change.  However, I would not be
surprised to see an argument being made that, when constructing
ClassProperties for a class, that we should not only examine super classes
for annotations, but also interfaces.

My personal opinion is that the pojomator for a class should not include
properties annotated on any of interfaces which the class implements.  If an
interface wishes to specify how equals and hashCode should be implemented,
it can do so as described in the first use case above.  But in general, a
class implementation should not need to worry about what annotations might
or might not be present on its interfaces.

What do others think about this?  Should the pojomator for a class include
annotated properties from interfaces or not?

  - Ian