This paper reports on the development of a language construct designed to solve certain problems in composability at the level of object-oriented programming languages. Features were chosen to investigate how much compositional functionality could be added to core Java with as small a change as possible and in an additive manner. A fairly elegant and effective syntax on core Java resulted in reducing the clutter of the ''mechanics'' of composition. This construct features eliminating runtime cast and null exceptions within the construct, and the option of parametric covariant override. It does so while avoiding undesirable restrictions or new frameworks. Recently, new techniques intended to improve the composability of software components in the Java ecosystem have been vigorously pursued. Proposals take on the objectives of composition in largescale systems and enterprise applications but often implement on conventional object-oriented languages which face the same kinds of problems from the ground up. Benefitting from those experiences, the present work seeks to provide core Java-level solutions to problems that are analogous to those at the higher-levels. The supervenience construct described in this paper is based on a well-established formulation for composing a relationship over classes with some common behavioral elements, their behavioral intersection. The narrow concern of this construct is the sound composition of components across class families while eliminating certain barriers to runtime type-safety in core Java. The supervenience construct has been applied to certain classic problems and key case studies in composability and extensibility which represent fundamental issues that emerge at every level. The implementation requires no installed runtime frameworks and no changes to the JVM or type system of core Java. It is designed to deploy stand-alone or later integrate with the standard Java compiler and IDEs. The continuing research is now focusing on a more formal proof of its type-safety, applying the reference implementation to mid-scale and distributed applications.