This thesis defines a set of program restructuring operations (refactorings) that support the design, evolution and reuse of object-oriented application frameworks. The focus of the thesis is on automating the refactorings in a way that preserves the behavior of a program. The refactorings are defined to be behavior preserving, provided that their preconditions are met. Most of the refactorings are simple to implement and it is almost trivial to show that they are behavior preserving. However, for a few refactorings, one or more of their preconditions are in general undecidable. Fortunately, for some cases it can be determined whether these refactorings can be applied safely. Three of the most complex refactorings are defined in detail: generalizing the inheritance hierarchy, specializing the inheritance hierarchy and using aggregations to model the relationships among classes. These operations are decomposed into more primitive parts, and the power of these operations is discussed from the perspectives of automatability and usefulness in supporting design. Two design constraints needed in refactoring are class invariants and exclusive components. These constraints are needed to ensure that behavior is preserved across some refactorings. This thesis gives some conservative algorithms for determining whether a program satisfies these constraints, and describes how to use this design information to refactor a program.

We propose design patterns as a new mechanism for expressing object-oriented design experience. Design patterns identify, name, and abstract common themes in objectoriented design. They capture the intent behind a design by identifying objects, their collaborations, and the distribution of responsibilities. Design patterns play many roles in the object-oriented development process: they provide a common vocabulary for design, they reduce system complexity by naming and de ning abstractions, they constitute a base of experience for building reusable software, and they act as building blocks from which more complex designs can be built. Design patterns can be considered reusable micro-architectures that contribute to an overall system architecture. We describe how to express and organize design patterns and introduce a catalog of design patterns. We also describe our experience in applying design patterns to the design of object-oriented systems.

We propose a new model for flexible composition of objects from a set of features. Features are similar to (abstract) subclasses, but only provide the core functionality of a (sub)class. Overwriting other methods is viewed as resolving feature interactions and is specifi separately for two features at a time. This programming model allows to compose features (almost) freely in a waywhich generalizes inheritance and aggregation. For a set of n features, an exponential number of different feature combinations is possible, assuming a quadratic number of interaction resolutions. We present the feature model as an extension of Java and givetwo translations to Java, one via inheritance and the other via aggregation. We further discuss parameterized features, which work nicely with our feature model and can be translated into Pizza, an extension of Java.

. A critical concern in the reuse of software is the propagation of changes made to reusable artifacts. Without techniques to manage these changes, multiple versions of these artifacts will propagate through different systems and reusers will not be able to benefit from improvements to the original artifact. We propose to codify the management of change in a software system by means of reuse contracts that record the protocol between managers and users of a reusable asset. Just as real world contracts can be extended, amended and customised, reuse contracts are subject to parallel changes encoded by formal reuse operators: extension, refinement and concretisation. Reuse contracts and their operators serve as structured documentation and facilitate the propagation of changes to reusable assets by indicating how much work is needed to update previously built applications, where and how to test and how to adjust these applications. 1 Introduction It has become a well-known fact that the...

Maintenance tends to degrade the structure of software, ultimately making maintenance more costly. At times, then, it is worthwhile to manipulate the structure of a system to make changes easier. However, it is shown that manual restructuring is an error-prone and expensive activity. By separating structural manipulations from other maintenance activities, the semantics of a system can be held constant by a tool, assuring that no errors are introduced by restructuring. To allow the maintenance team to focus on the aspects of restructuring and maintenance requiring human judgment, a transformation-based tool can be provided---based on a model that exploits preserving data flow-dependence and control flow-dependence---to automate the repetitive, errorprone, and computationally demanding aspects of re...

Reusing the products of the software development process is an important way to reduce software costs and to make programmers and designers more efficient. Object-oriented programming permits the reuse of design as well as programs. This paper describes two techniques for reusing design and how these reusable designs are developed. Abstract classes are reusable designs for components while frameworks are reusable designs for entire applications or subsystems. These two techniques are related since frameworks almost always contain abstract classes. Although the most widely used frameworks are for user interfaces, this paper draws its examples from a framework for the virtual memory subsystem of an operating system.

Program refactoring --- transforming a program to improve readability, structure, performance, abstraction, maintainability, or other characteristics --- is not applied in practice as much as might be desired. One deterrent is the cost of detecting candidates for refactoring and of choosing the appropriate refactoring transformation. This paper demonstrates the feasibility of automatically finding places in the program that are candidates for specific refactorings. The approach uses program invariants: when particular invariants hold at a program point, a specific refactoring is applicable. Since most programs lack explicit invariants, an invariant detection tool called Daikon is used to infer the required invariants. We developed an invariant pattern matcher for several common refactorings and applied it to an existing Java code base. Numerous refactorings were detected, and one of the developers of the code base assessed their efficacy.

Object-oriented programs evolve by means other than just the addition of new classes. The changes to object-oriented programs that have been most studied are those based on inheritance, on reorganizing a class hierarchy. However, aggregation is a relationship between classes that is just as important as inheritance, and many changes to an object-oriented design involve the aggregate/component relationship. This paper describes some common refactorings based on aggregation, including how to convert from inheritance to an aggregation, and how to reorganize an aggregate/component hierarchy just as one might reorganize a class inheritance hierarchy. 1 Introduction Object-oriented programming is advertised as leading to more extensible programs than conventional programming [Mey88]. Modifications to a system that would require many changes in conventional programming often can be carried out in an object-oriented program by simply adding new classes. Nevertheless, there are many t...

Schema evolution is a problem that is faced by long-lived data. When a schema changes, existing persistent data can become inaccessible unless the database system provides mechanisms to access data created with previous versions of the schema. Most existing systems that support schema evolution focus on changes local to individual types within the schema, thereby limiting the changes that the database maintainer can perform. We have developed a model of type changes incorporating changes local to individual types as well as compound changes involving multiple types. The model describes both type changes and their impact on data by defining derivation rules to initialize new data based on the existing data. The derivation rules can describe local and nonlocal changes to types to capture the intent of a large class of type change operations. We have built a system called Tess (Type Evolution Software System) that uses this model to recognize type changes by comparing schemas and then produces a transformer that can update data in a database to correspond to a newer version of the schema.

Building and maintaining the class hierarchy has been recognized as an important but one of the most difficult activities of object-oriented design. Concept (or Galois) lattices and related structures are presented as a framework for dealing with the design and maintenance of class hierarchies. Because the design of class hierarchies is inherently an iterative and incremental process, we designed incremental algorithms that update existing Galois lattices as the result of adding, removing, or modifying class specifications. A prototype tool incorporating this and other algorithms has been developed as part of the IGLOO project, which is a large object-oriented software engineering joint research project involving academic and industrial partners. The tool can generate either the concept lattice or several variant structures incrementally by incorporating new classes one by one. The resulting hierarchies can be interactively explored and refined using a graphical browser. In addition, s...