I am a big fan of the APR versioning guidelines, but there is an element they don't capture well somewhere between major (backwards incompat change) and minor (forward incompat change) in Java. If you follow the, generally recommended practice of exposing things via interfaces (pure virtual classes), you have opened the door for users to implement those interfaces.
In a C-style world, adding a function to a library would bump you from 1.6 to 1.7, using APR guidelines. In an interface-driven Java-style world, adding a method to an interface would bump you from 1.6 to 2.0. Or would it?
To take a concrete example, a coworker (thanks Jax!) recently
re-added first class support for callable statements to jDBI. jDBI
uses a Handle interface to expose operations against a
database. It has gained a method:
public <ReturnType> Call<ReturnType> createCall(String callableSql,
CallableStatementMapper<ReturnType> mapper);
If you implement this interface, the change is backwards
incompatible. An implementation of Handle made against
2.2.2 will not compile against this. On the other hand, the intent
of the library is not for people to implement Handle,
it is to expose the libraries functionality. It is almost a
header file.
So, 2.3 or 3.0?