Interfaces as defined in the JAVA programming language can enhance both decoupling and comprehensibility of large code bases. Several researchers have pointed out this key role of interfaces in object-oriented programming, but so far only little insight as to how interfaces are actually used in practice has been made available. We fill this gap by applying a special metrics suite to one of the most popular pieces of software, the JAVA DEVELOPMENT KIT, and present interesting results. Categories and Subject Descriptors D.3.3 [Programming Languages]: Language Constructs and – Abstract data types, Classes and objects, Data types and structures, Frameworks, Inheritance, Patterns, Polymorphism General Terms Measurement, Design, Experimentation, Languages. Keywords Metrics, Interfaces, JAVA, Frameworks, Refactorings.