How to avoid calling implements at 2 activites?

I have two activities: MainActivity extends ListActivity and Preferences implements BillingController.IConfiguration.

Now I need t开发者_JAVA技巧o call BillingController.someMethod(this) at MainActivity. Such call can be made only if MainActivity implements BillingController.IConfiguration. But this is already implemented at Preferences class. What can I do to avoid implementing that again?

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Make a class that implements BillingController.IConfiguration and use it inside both MainActivity and Preferences to delegate method calls to it.

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Do an instance of check.

if (instance instanceof BillingController.IConfiguration) {
    ((BillingController.IConfiguration)instance).someMethod(this);
}