Inge Software

Software Quality Metrics for Object-Oriented Environments
AUTHORS: Dr. Linda H. Rosenberg Unisys Government Systems Goddard Space Flight Center Bld 6 Code 300.1 Greenbelt, MD 20771 USA Lawrence E. Hyatt Software Assurance Technology Center Goddard Space Flight Center Bld 6 Code 302 Greenbelt, MD 20771 USA

I. INTRODUCTION Object-oriented design and development are popular concepts in today’ssoftware development environment. They are often heralded as the silver bullet for solving software problems. While in reality there is no silver bullet, object-oriented development has proved its value for systems that must be maintained and modified. Object-oriented software development requires a different approach from more traditional functional decomposition and data flow development methods.This includes the software metrics used to evaluate object-oriented software. The concepts of software metrics are well established, and many metrics relating to product quality have been developed and used. With object-oriented analysis and design methodologies gaining popularity, it is time to start investigating object-oriented metrics with respect to software quality. We are interested in theanswer to the following questions: • • • • What concepts and structures in object-oriented design affect the quality of the software? Can traditional metrics measure the critical object-oriented structures? If so, are the threshold values for the metrics the same for object-oriented designs as for functional/data designs? Which of the many new metrics found in the literature are useful to measurethe critical concepts of object-oriented structures?

II. METRIC EVALUATION CRITERIA While metrics for the traditional functional decomposition and data analysis design approach measure the design structure and/or data structure independently, object-oriented metrics must be able to focus on the combination of function and data as an integrated object [1]. The evaluation of the utility of ametric as a quantitative measure of software quality was based on the measurement of a software quality attribute. The metrics selected, however, are useful in a wide range of models. The object-oriented metric criteria, therefore, are to be used to evaluate the following attributes: • • • Efficiency - Are the constructs efficiently designed? Complexity - Could the constructs be used more effectivelyto decrease the architectural complexity? Understandability - Does the design increase the psychological complexity?

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Reusability - Does the design quality support possible reuse? Testability/Maintainability - Does the structure support ease of testing and changes?

Whether a metric is “traditional” or “new”, it must be effective in measuring one or more of these attributes.As each metric is presented, we will briefly discuss its applicability. The SATC’s approach to identifying a set of object-oriented metrics was to focus on the primary, critical constructs of object-oriented design and to select metrics that apply to those areas. The suggested metrics are supported by most literature and some object-oriented tools. The metrics evaluate the object-orientedconcepts: methods, classes, coupling, and inheritance. The metrics focus on internal object structure, external measures of the interactions among entities, measures of the efficiency of an algorithm and the use of machine resources, as well as psychological measures that affect the ability of a programmer to create, comprehend, modify, and maintain software. We support the use of three traditionalmetrics and present six additional metrics specifically for object-oriented systems. The SATC has found that there is considerable disagreement in the field about software quality metrics for object-oriented systems [1,4]. Some researchers and practitioners contend traditional metrics are inappropriate for object-oriented systems. There are valid reasons for applying traditional metrics, however, if...