Classless Java Interface-Based Programming for the Masses

This paper introduces interface-based object-oriented programming (IB). Unlike class-based OO programming (CB), in IB there is no need for classes. To support state, IB uses abstract state operations, such as getters, setters or clone-like methods. This new way to deal with state allows flexibility not typically available in CB. In IB state (including mutable state) can be type-refined in subtypes. The combination of a purely IB style of programming and type-refinement enables powerful idioms using multiple inheritance and state. To introduce IB to programmers we created Classless Java: an embedding of IB directly into Java. Classless Java uses annotation processing for code generation, and relies on new features of Java 8 for interfaces. The code generation techniques used in Classless Java have interesting properties, including guarantees that no type errors occur in generated code and good integration with IDE’s. We illustrate the usefulness of IB and Classless Java through various examples and case studies.