We present a domain-independent model of hierarchical software system design and construction that is based on interchangeable software components and largescale reuse. The model unifies the conceptualizations of two independent projects, Genesis and Avoca, that are successful examples of software component/building-block technologies and domain modeling. Building-block technologies exploit large-scale reuse, rely on open architecture software, and elevate the granularity of programming to the subsystem level. Domain modeling formalizes the similarities and differences among systems of a domain. We believe our model is a blue-print for achieving software component technologies in many domains.

Previous work on type-theoretic foundations for object-oriented programming languages has mostly focused on applying or extending functional type theory to functional "objects." This approach, while benefiting from a vast body of existing literature, has the disadvantage of dealing with state change either in a roundabout way or not at all, and completely sidestepping issues of concurrency. In particular, dynamic issues of non-uniform service availability and conformance to protocols are not addressed by functional types. We propose a new type framework that characterizes objects as regular (finite state) processes that provide guarantees of service along public channels. We also propose a new notion of subtyping for active objects, based on Brinksma's notion of extension, that extends Wegner and Zdonik's "principle of substitutability" to non-uniform service availability. Finally, we formalize what it means to "satisfy a client's expectations," and we show how regular types canbe used...

Abstract. Mixin layers are a technique for implementing layered object-oriented subclasses (mixin classes) but scaled to a multiple-class granularity. We describe mixin layers from a programming language viewpoint, discuss checking the consistency of a mixin layer composition, and analyze the language support issues involved. 1

A "refinement" is a functionality addition to a software project that can affect multiple dispersed implementation entities (functions, classes, etc.). In this paper, we examine large-scale refinements in terms of a fundamental object-oriented technique called collaboration-based design. We explain how collaborations can be expressed in existing programming languages or be supported with new language constructs (which we have implemented as extensions to the Java language). We present a specific expression of large-scale refinements called mixin layers, and demonstrate how it overcomes the scalability difficulties that plagued prior work. We also show how we used mixin layers as the primary implementation technique for building an extensible Java compiler, JTS.

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Multi-methods allow method selection to be based on the types of any number of arguments. Languages that currently support multi-methods do not support static type checking. We show how multi-methods can be statically type checked and how information collected at the time of program compilation can be used to make the run-time dispatch of multi-methods more efficient. The results presented can provide the basis for introducing multi-methods in languages with static type checking and for designing new object-oriented paradigms based on multi-methods.

We present a framework to extend the concept of type systems in programming languages to capture the dynamic interaction in component-based design, such as the communication protocols between components. In our system, the interaction types and the dynamic behavior of components are defined using interface automata - an automata-based formalism. Type checking, which checks the compatibility of a component with a certain interaction type, is conducted through automata composition. Our type system is polymorphic in that a component may be compatible with more than one interaction type. We show that a subtyping relation exists among various interaction types and this relation can be described using a partial order. This system-level type order can be used to facilitate the design of polymorphic components and simplify type checking.

We investigate the use of the dot notation in the context of abstract types. The dot notation—that is, a.f referring to the operation f provided by the abstraction a—is used by programming languages such as Modula-2 and CLU. We compare this notation with the Mitchell-Plotkin approach, which draws a parallel between type abstraction and (weak) existential quantification in constructive logic. The basic operations on existentials coming from logic give new insights about the meaning of type abstraction, but differ completely from the more familiar dot notation. In this paper, we formalize simple calculi equipped with the dot notation, and relate them to a more classical calculus à la Mitchell and Plotkin. This work provides some theoretical foundations for the dot notation, and suggests some useful extensions.

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Following our own experience developing a concurrent object-oriented language as well of that of other researchers, we have identb))1 several key problems intT design of a concurrency model compat1}+ wit t he mechanisms ofobject}+4gTT1t programming. We propose an approacht language design in which anexecut1}+ notcut describingte behaviour ofcommunicatTT agent isext+T5+ bysynt4g81 pat4g8 tt encapsulat languageconste1T)+ Weindicat how various language models can beaccommodat5T and how mechanisms such as inherit}+5 can be modeled. Finally, weint6+TT1 a newnotb( oft ypest hat charact(1}T6 concurrent object in t1+6 oft15+ ext5g41}T visible behaviour. 1.