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

The '''NonMixedFactory.Provider''' interface would then become:

<source lang="java">
public abstract class Provider {
protected abstract void initialize(Callback c);
protected abstract int toBeImplementedBySubclass();
}
</source>

It is clear that these '''protected''' method cannot be called by anyone else except the [[API]] infrastructure. On the othe[[Category:APIDesignPatterns]]
[[Category:APIDesignPatterns:Clarity]]
[[Category:APIDesignPatterns:Creational]]
r hand, this prevents multiple inheritance for implementors. Whether to choose the [[ImplementOnlyAbstractClasses]] or [[ImplementOnlyInterface]] is ultimately a matter of personal preference.