Frank Tip

Reengineering class hierarchies using concept analysis

The design of a class hierarchy may be imperfect. For example, a class C may contain a member m not accessed in any C-instance, an indication that m could be eliminated, or moved into a derived class. Furthermore, different subsets of Cs members may be accessed from different C-instances, indicating that it might be appropriate to split C into multiple classes. We present a framework for detecting and remediating such design problems, which is based on concept analysis. Our method analyzes a class hierarchy along with a set of applications that use it, and constructs a lattice that provides valuable insights into the usage of the class hierarchy in a specific context. We show how a restructured class hierarchy can be generated from the lattice, and how the lattice can serve as a formal basis for interactive tools for redesigning and restructuring class hierarchies.

Refactoring support for class library migration

As object-oriented class libraries evolve, classes are occasionally deprecated in favor of others with roughly the same functionality. In Javas standard libraries, for example, class Hashtable has been superseded by HashMap, and Iterator is now preferred over Enumeration. Migrating client applications to use the new idioms is often desirable, but making the required changes to declarations and allocation sites can be quite labor-intensive. Moreover, migration becomes complicated---and sometimes impossible---if an application interacts with external components, if a legacy class is not completely equivalent to its replacement, or if multiple interdependent classes must be migrated simultaneously. We present an approach in which mappings between legacy classes and their replacements are specified by the programmer. Then, an analysis based on type constraints determines where declarations and allocation sites can be updated. The method was implemented in Eclipse, and evaluated on a number of Java applications. On average, our tool could migrate more than 90% of the references to legacy classes.

Refactoring for generalization using type constraints

Refactoring is the process of applying behavior-preserving transformations (called refactorings) in order to improve a programs design. Associated with a refactoring is a set of preconditions that must be satisfied to guarantee that program behavior is preserved, and a set of source code modifications. An important category of refactorings is concerned with generalization (e.g., Extract Interface for re-routing the access to a class via a newly created interface, and Pull Up Members for moving members into a superclass). For these refactorings, both the preconditions and the set of allowable source code modifications depend on interprocedural relationships between types of variables. We present an approach in which type constraints are used to verify the preconditions and to determine the allowable source code modifications for a number of generalization-related refactorings. This work is implemented in the standard distribution of Eclipse (see www.eclipse.org).

Understanding class hierarchies using concept analysis

A new method is presented for analyzing and reengineering class hierarchies. In our approach, a class hierarchy is processed along with a set of applications that use it, and a fine-grained analysis of the access and subtype relationships between objects, variables, and class members is performed. The result of this analysis is again a class hierarchy, which is guaranteed to be behaviorally equivalent to the original hierarchy, but in which each object only contains the members that are required. Our method is semantically well-founded in concept analysis: the new class hierarchy is a minimal and maximally factorized concept lattice that reflects the access and subtype relationships between variables, objects and class members. The method is primarily intended as a tool for finding imperfections in the design of class hierarchies, and can be used as the basis for tools that largely automate the process of reengineering such hierachies. The method can also be used as a space-optimizing source-to-source transformation that removes redundant fields from objects. A prototype implementation for Java has been constructed, and used to conduct several case studies. Our results demonstrate that the method can provide valuable insights into the usage of a class hierarchy in a specific context, and lead to useful restructuring proposals.

Slicing class hierarchies in C

This paper describes an algorithm for slicing class hierarchies in C++ programs. Given a C++ class hierarchy (a collection of C++ classes and inheritance relations among them) and a program P that uses the hierarchy, the algorithm eliminates from the hierarchy those data members, member functions, classes, and inheritance relations that are unnecessary for ensuring that the semantics of P is maintained.Class slicing is especially useful when the program P is generated from a larger program P by a statement slicing algorithm. Such an algorithm eliminates statements that are irrelevant to a set of slicing criteria---program points of particular interest. There has been considerable previous work on statement slicing, and it will not be the concern of this paper. However, the combination of statement slicing and class slicing for C++ has two principal applications: First, class slicing can enhance statement slicings utility in program debugging and understanding applications, by eliminating both executable and declarative program components irrelevant to the slicing criteria. Second, the combination of the two slicing algorithms can be used to decrease the space requirements of programs that do not use all the components of a class hierarchy. Such a situation is particularly common in programs that use class libraries.

Refactoring for Parameterizing Java Classes

Type safety and expressiveness of many existing Java libraries and their client applications would improve, if the libraries were upgraded to define generic classes. Efficient and accurate tools exist to assist client applications to use generic libraries, but so far the libraries themselves must be parameterized manually, which is a tedious, time-consuming, and error-prone task. We present a type- constraint-based algorithm for converting non-generic libraries to add type parameters. The algorithm handles the full Java language and preserves backward compatibility, thus making it safe for existing clients. Among other features, it is capable of inferring wildcard types and introducing type parameters for mutually-dependent classes. We have implemented the algorithm as a fully automatic refactoring in Eclipse. We evaluated our work in two ways. First, our tool parameterized code that was lacking type parameters. We contacted the developers of several of these applications, and in all cases they confirmed that the resulting parameterizations were correct and useful. Second, to better quantify its effectiveness, our tool parameterized classes from already-generic libraries, and we compared the results to those that were created by the libraries authors. Our tool performed the refactoring accurately-in 87% of cases the results were as good as those created manually by a human expert, in 9% of cases the tool results were better, and in 4% of cases the tool results were worse.

Refactoring using type constraints

Type constraints express subtype relationships between the types of program expressions, for example, those relationships that are required for type correctness. Type constraints were originally proposed as a convenient framework for solving type checking and type inference problems. This paper shows how type constraints can be used as the basis for practical refactoring tools. In our approach, a set of type constraints is derived from a type-correct program P. The main insight behind our work is the fact that P constitutes just one solution to this constraint system, and that alternative solutions may exist that correspond to refactored versions of P. We show how a number of refactorings for manipulating types and class hierarchies can be expressed naturally using type constraints. Several refactorings in the standard distribution of Eclipse are based on our work.

A study of dead data members in C++ applications

Object-oriented applications may contain data members that can be removed from the application without affecting program behavior. Such dead data members may occur due to unused functionality in class libraries, or due to the programmer losing track of member usage as the application changes over time. We present a simple and efficient algorithm for detecting dead data members in C++ applications. This algorithm has been implemented using a prototype version of the IBM VisualAge C++ compiler, and applied to a number of realistic benchmark programs ranging from 600 to 58,000 lines of code. For the non-trivial benchmarks, we found that up to 27.3% of the data members in the benchmarks are dead (average 12.5%), and that up to 11.6% of the object space of these applications may be occupied by dead data members at run-time (average 4.4%).

Extracting library-based object-oriented applications

In an increasingly popular model of software distribution, software is developed in one computing environment and deployed in other environments by transfer over the internet. Extraction tools perform a static whole-program analysis to determine unused functionality in applications in order to reduce the time required to download applications. We have identified a number of scenarios where extraction tools require information beyond what can be inferred through static analysis: software distributions other than complete applications, the use of reflection, and situations where an application uses separately developed class libraries. This paper explores these issues, and introduces a modular specification language for expressing the information required for extraction. We implemented this language in the context of Jax, an industrial-strength application extractor for Java, and present a small case study in which different extraction scenarios are applied to a commercially available library-based application.

Class hierarchy specialization

Abstract. Many class libraries are designed with an emphasis on generality and extensibility. Applications often exercise only part of a librarys functionality. As a result, the objects created by an application may contain unused (user-specified or compiler-generated) members. Redundant members in objects are undesirable because they increase an applications memory usage.We present an algorithm for specializing a class hierarchy with respect to its usage in a program n