Karl J. Lieberherr

Assuring good style for object-oriented programs

The language-independent Law of Demeter, which encodes the ideas of encapsulation and modularity in an easy-to-follow form for object-oriented programmers, is presented. The law was developed during the design and implementation of the Demeter system, which provides a high-level interface to class-based, object-oriented systems. Two forms of the law, the class and object forms, are described. Its motivation is to ensure that the software is as modular as possible. Principles covered by the law include coupling control, information hiding, information restriction, information localization, and structured induction. An example is given to show how the law is applied, and valid violations are identified. It is shown how to transform a method that does not satisfy the law into one that does.<<ETX>>

Discussing aspects of AOP

Aspect-oriented programming is a new evolution in the line of technology for separation of concerns technology that allows design and code to be structured to reflect the way developers want to think about the system. AOP builds on existing technologies and provides additional mechanisms that make it possible to affect the implementation of systems in a crosscutting way.Aspect-oriented programming is a new evolution in the line of technology for separation of concerns technology that allows design and code to be structured to reflect the way developers want to think about the system. AOP builds on existing technologies and provides additional mechanisms that make it possible to affect the implementation of systems in a crosscutting way.

Aspect-oriented programming with adaptive methods

A An operation in an object-oriented program often involves several different collaborating classes. There are usually two ways to implement such an operation: either put the whole operation into one method on one of the classes, or divide the operation into methods on each of the classes involved. The drawback of the former is that too much information about the structure of the classes (is-a and has-a relationships) needs to be tangled into each such method, making it difficult to adapt to changes in the class structure. However, the latter scatters the operation across multiple classes, making it difficult to adapt when the operation changes.

Object-oriented programming: an objective sense of style

We introduce a simple, programming language independent rule (known in-house as the Law of Demeter™) which encodes the ideas of encapsulation and modularity in an easy to follow form for the object-oriented programmer. You tend to get the following related benefits when you follow the Law of Demeter while minimizing simultaneously code duplication, the number of method arguments and the number of methods per class: Easier software maintenance, less coupling between your methods, better information hiding, narrower interfaces, methods which are easier to reuse, and easier correctness proofs using structural induction. We discuss two important interpretations of the Law (strong and weak) and we prove that any object-oriented program can be transformed to satisfy the Law. We express the Law in several languages which support object-oriented programming, including Flavors, Smalltalk-80, CLOS, C++ and Eiffel.

Aspectual Collaborations: Combining Modules and Aspects

Complex behavior often resists clean modularization in object-oriented languages. Aspectoriented programming tackles this problem by providing ¤exible module boundaries that can span and partition classes. However, in order to achieve this ¤exibility, valuable modularity mechanisms, such as encapsulation and external composition, are lost. The ability to separately compile or reason about a modular unit is also compromised. We propose that this tradeoff is not necessary, and that by expressing aspects as part of our modules, we can restore lost modular properties while maintaining aspectual features. As a concrete demonstration, we present the main features of Aspectual Collaborations, and show how these interact to combine modularity with aspectual behavior. The expressiveness of Aspectual Collaborations, AspectJ, and Hyper/J are compared using a challenge problem, allowing us to estimate the effectiveness of the approach.

DJ: Dynamic Adaptive Programming in Java

DJ is a new pure-Java library for adaptive programming that allows traversal strategies to be constructed and in terpreted dynamically at run-time, as opposed to existing adaptive programming tools that are strictly static. Traversal strategies can be used with adaptive visitors (similar to the Visitor pattern) or in a generic programming style by adapting them to the Java Collections framework. The DJ library makes heavy use of Java reflection and we give some details of this implementation.

Adaptive object-oriented programming using graph-based customization

Object-oriented programs are easier to extend than programs which are not written in an object-oriented style, but object-oriented programs are still very rigid and hard to adapt and maintain. In this article, we introduce adaptive object-oriented programming as an extension to conventional object-oriented programming. Adaptive object-oriented programming facilitates expressing the elements-classes and methods-that are essential to an application by avoiding to make a commitment on the particular class structure of the application. Adaptive programs are speciied using propagation patterns which specify sets of related constraints on class structures. An adaptive program denotes an entire family of programs, as many programs as there are class structures which satisfy its constraints. A class structure which satisses the constraints of an adaptive program is said to customize the program. Adaptive programming, realized by the use of propagation patterns, ooers a new paradigm, extending the object-oriented paradigm by lifting programming to a higher level of abstraction.

Traversals of object structures: Specification and Efficient Implementation

Separation of concerns and loose coupling of concerns are important issues in software enginnering. In this paper we show how to separate traversal-related concerns from other concerns, how to loosely couple traversal-related concerns to the structural concern, and how to efficiently implement traversal-related concerns. The stress is on the detailed description of our algorithms and the traversal specifications they operate on.Traversal of object structures is a ubiquitous routine in most types of information processing. Ad hoc implementations of traversals lead to scattered and tangled code and in this paper we present a new approach, called traversal strategies, to succinctly modularize traversals. In our approach traversals are defined using a high-level directed graph description, which is compiled into a dynamic road map to assist run-time traversals. The complexity of the compilation algorithm is polynomial in the size of the traversal strategy graph and the class graph of the given application. Prototypes of the system have been developed and are being successfully used to implement traversals for Java and AspectJ [Kiczales et al. 2001] and for generating adapters for software components. Our previous approach, called traversal specifications [Lieberherr 1992; Palsberg et al. 1995], was less general and less succinct, and its compilation algorithm was of exponential complexity in some cases. In an additional result we show that this bad behavior is inherent to the static traversal code generated by previous implementations, where traversals are carried out by invoking methods without parameters.

Component Integration with Pluggable Composite Adapters

In this chapter we address object-oriented component integration issues. We argue that traditional framework customization techniques are inappropriate for component-based programming since they lack support for non-invasive, encapsulated, dynamic customization. We propose a new language construct, called a pluggable composite adapter, for expressing component gluing. A pluggable composite adapter allows the separation of customization code from component implementation, resulting in better modularity, flexible extensibility, and improved maintenance and understandability. We also discuss alternative realizations of the construct.

XAspects: an extensible system for domain-specific aspect languages

Current general aspect-oriented programming solutions fall short of helping the problem of separation of concerns for several concern domains. Because of this limitation good solutions for these concern domains do not get used and the opportunity to benefit from separation of these concerns is missed. By using XAspects, a plug-in mechanism for domain-specific aspect languages, separation of concerns can be achieved at a level beyond what is possible for object-oriented programming languages. As a result, XAspects allows for certain domain-specific solutions to be used as easily as a new language feature.