Intersection Types in Java: Back to the Future

In this paper we figure out the future of intersection types in Java developments, based both on the primary meaning of the intersection type constructor and on the present approach in Java. In our vision, the current use of intersection types will be extended in two directions. Firstly, intersections will be allowed to appear as types of fields, types of formal parameters and return values of methods, therefore they will be significantly used as target types for expressions anywhere. Secondly, the notion of functional interface will be extended to any intersection of interfaces, including also several abstract methods with different signatures. Thus a single target type will be able to express multiple, possibly unrelated, properties of one expression. We formalise our proposal through a minimal Java core extended with these novel features and we prove the type safety property.