Design Issues in Concurrent Object-Oriented Languages and Observability

This paper discusses different choices in the design of object-oriented, concurrent language from the perspective of observability. Observability takes the standpoint that two ``program fragments´´ are observably equivalent if one can be replaced by the other without leading to differences in a larger context. Characterizing the observable behavior of a program fragment is therefore crucial for compositionality. The choice of language constructs has a big impact on what can be observed, and thus also how well-suited the language is for being composed. In this paper, we concentrate on well-established variants of constructs in object-oriented languages and discuss their influence on the observable semantics. In particular, we discuss classes as units of code, inheritance as the mainstream way of code re-use in class-based, object-oriented languages. For concurrency, we compare the two most prominent ways to combine objects and concurrency: multi-threading as for instance done in Java vs. the active objects or actor paradigm. A final aspect is the influence of the synchronization mechanism of locks and monitors.