ImplementOnlyAbstractClass

From APIDesign

Safer variant to ImplementOnlyInterface is to use abstract class with protected methods. Then it is clear the meaning of such methods is clear again as advocated by ClarityOfAccessModifiers.

The NonMixedFactory.Provider interface would then become:

public abstract class Provider {
  protected abstract void initialize(Callback c);
  protected abstract int toBeImplementedBySubclass();
}

It is clear that these protected method cannot be called by anyone else except the API infrastructure. On the other hand, this prevents multiple inheritance for implementors and delegation to other implementors. Whether to choose the ImplementOnlyAbstractClass or ImplementOnlyInterface is ultimately a matter of personal preference.