Auxiliary for Optional.absent()

[jeremyjliu]

Say I had an object that I use as a key in map:

@Value.Immutable
public interface Key() {
  String key()
}

Then at some point in the future, I add a field to the key:

@Value.Immutable
public interface Key() {
  String key()
  Optional<String> optionalKey()
}

But, say I was intent on keeping backwards compatibility with the existing keys in the map. Since the new field is an Optional, how could I do this? By default, the hashCode for the new field is factored into Key.hashCode() even if the optionalKey().hashCode() == 0 due to the left shift.

In essence, I would want the optionalKey() to be auxiliary if it was absent and non-auxiliary if the value is present.