The Problem: Evolving Interfaces Without Breaking Implementations Default methods solved a problem that had blocked the Java Collections team from improving the core APIs for years: how do you add methods to an interface without breaking every existing implementation? The answer Java 8 gave — concrete implementations on the interface itself — has wider implications than just Collection.stream(). It is the mechanism behind the fluent Comparator API, the Predicate composition methods, and the design pattern of composable interface contracts.