When should internal interfaces be promoted to public?

Commonly, software systems have public (and stable) interfaces, and internal (and possibly unstable) interfaces. Despite being discouraged, client developers often use internal interfaces, which may cause their systems to fail when they evolve. To overcome this problem, API producers may promote internal interfaces to public. In practice, however, API producers have no assistance to identify public interface candidates. In this paper, we study the transition from internal to public interfaces. We aim to help API producers to deliver a better product and API clients to benefit sooner from public interfaces. Our empirical investigation on five widely adopted Java systems present the following observations. First, we identified 195 promotions from 2,722 internal interfaces. Second, we found that promoted internal interfaces have more clients. Third, we predicted internal interface promotion with precision between 50%-80%, recall 26%-82%, and AUC 74%-85%. Finally, by applying our predictor on the last version of the analyzed systems, we automatically detected 382 public interface candidates.