SOFTWARE QUALITY METRICS

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SOFTWARE QUALITY METRICS

for

Object Oriented

System Environments

SATC-TR-95-1001

JUNE 1995

National Aeronautics and Space Administration

Goddard Space Flight Center, Greenbelt Maryland 20771

Software

Assurance

Technology

Center

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TABLE OF CONTENTS

I. Introduction

II. Metrics Overview

III. Object-Oriented Overview

IV. Metrics for Object-Oriented Systems

A. Metric Evaluation Criteria

B. Traditional Metrics for Object-Oriented Systems

1. Methods

Metric 1: Cyclomatic Complexity

Metric 2: Lines of Code

C. Object-Oriented Specific Metrics

1. Classes

a. Methods

Metric 3: Weighted Methods per Class (WMC)

b. Messages

Metric 4: Response for a Class (RFC)

c. Cohesion

Metric 5: Lack of Cohesion of Methods (LCOM)

d. Coupling

Metric 6: Coupling Between Object Classes (CBO)

2. Inheritance

Metric 7: Depth of Inheritance Tree (DIT)

Metric 8: Number of Children (NOC)

D. Example

E. Summary

1. Metric Summary

2. COTS for Recommended Object-Oriented Metrics

V. Conclusions

Appendix A: Comprehensive Listing of Object-Oriented Metrics

Appendix B: Object-Oriented Detailed Discussion

1. Object

a. States/Attributes

b. Operations

c. Object Example

2. Class

3. Inheritance

4. Messages

5. Cohesion

6. Polymorphism

7. Object-Oriented Languages

8. Terminology

Appendix C: Cots Packages

Appendix D: Object-Oriented References

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TABLES

Table 1: SATC Metrics for Object-oriented Systems

Table 2: Key Object-Oriented Definitions

Table 3: Summary of Recommended Object-Oriented Metrics

Table 4: Object-Oriented Metrics Supported by COTS

FIGURES

Figure 1: Pictorial Description of Key Object-Oriented Terms

Figure 2: Geometric Classes with Attribute, Operations and Methods

Figure 3: Pseudocode for Perimeter of an Equilateral Triangle

Figure 4: Notation of an Object

Figure 5: Sample Object

Figure 6: Conceptual View

Figure 7: Class with Objects

Figure 8: Inheritance Network

Figure 9: Message Passing

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Software Quality Metrics for Object-Oriented System Environments

I. INTRODUCTION

Object-oriented analysis and design are popular concepts in today’s software development environment. They

are often heralded as the silver bullet for solving software problems,; while in reality there is no silver bullet,

object-oriented 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. While the functional and data analysis approaches commence by considering the

systems behavior and/or data separately; object-oriented analysis approaches the problem by looking or system

entities that combine them. Object-oriented analysis and design focuses on objects as the primary agents

involved in a computation; each class of data and related operations are collected into a single system entity.

The concepts of software metrics are well established, and many metrics relating to product quality have been

developed and used. The SATC applies a model for evaluating software quality that has four goals:

(1) Stability of Requirements and Design, (2) Product Quality, (3) Testing Effectively, and

(4) Implementation Effectively. With object-oriented analysis and design methodologies gaining popularity, it

is time to start investigating object-oriented metrics with respect to these goals. We are interested in the

answer to the following questions:

· What concepts and structures in object-oriented 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 literature are useful to measure the critical concepts in

object-oriented?

This report summarizes results of the SATC’s research on metrics for object-oriented systems. We start with a

brief discussion of the metrics recommended by the SATC for object-oriented systems. These metrics include

modifications of “traditional” metrics as well as “new” metrics for specific object-oriented structures. Since the

object-oriented metrics require a cursory understanding of the object-oriented concepts, Section III presents a

pictorial representation of the basic object-oriented structures and defines the key terms. A more extensive

explanation of the object-oriented structures is in Appendix B and is referenced by Section III. In Section IV,

we discuss the proposed object-oriented metrics with respect to the SATC Software Quality Model,

specifically, their relationship to the attributes of quality (Goal 2: Product Quality -Structure/Architecture,

Reuse, Maintainability). In the summary, we will address the availability of COTS packages to facilitate the

collection of these metrics. Details on the COTS packages are given in Appendix C.

II. OVERVIEW - OBJECT-ORIENTED METRICS

In this report, the SATC documents its research into the current status of object-oriented metrics. The

research was done by surveying the literature on object-oriented metrics then applying the SATC experience in

traditional software metrics to select the object-oriented metrics that support the goal of measuring design and

code quality. In addition, we required that a metric be feasible and have a clear relationship to the objectoriented

structures being measured. At this time, many proposed object-oriented metrics lack a theoretical

basis and have not yet been validated. Some of these metrics are too labor intensive to collect, or are too

dependent on the implementation technology. The object-oriented metrics proposed by the SATC can be

related to desirable software qualities.

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

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supported by most literature and some object-oriented tools. The metrics evaluate the object-oriented concepts:

methods; classes; coupling; and inheritance. The metrics focus on internal object structure that reflects the

complexity of each individual entity and on external complexity that measures the interactions among entities.

The metrics measure computational complexity encompassing the efficiency of an algorithm and the use of

machine resources, as well as psychological complexity factors that affect the ability of a programmer to

create, modify, and comprehend software and the end user to effectively use the software.

We support the use of three traditional metrics and present six new 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. Some researchers and practitioners contend traditional metrics are

inappropriate for object-oriented systems. There are valid reasons for applying the traditional metrics

however, if it can be done. The traditional metrics have been widely used, they are well understood by

researchers and practitioners, and their relationships to software quality attributes have been validated.

Table 1 presents an overview of the metrics proposed by the SATC for object-oriented systems. The SATC

supports the continued use of traditional metrics, but within the structures and confines of object-oriented

systems. The first three metrics in Table 1 are examples of how traditional metrics can be applied to the

object-oriented structure of methods instead of functions or procedures. The next six metrics are specifically

for object-oriented systems and the object-oriented construct applicable is indicated.

SOURCE METRIC OBJECT-ORIENTED

CONSTRUCT

Traditional CC Cyclomatic complexity Method

Traditional SIZE Lines of Code Method

Traditional COM Comment percentage Method

NEW Object-Oriented WMC Weighted methods per class Class/Method

NEW Object-Oriented RFC Response for a class Class/Message

NEW Object-Oriented LCOM Lack of cohesion of methods Class/Cohesion

NEW Object-Oriented CBO Coupling between objects Coupling

NEW Object-Oriented DIT Depth of inheritance tree Inheritance

NEW Object-Oriented NOC Number of children Inheritance

Table 1: SATC Metrics for Object-Oriented Systems

III. OVERVIEW - OBJECT-ORIENTED STRUCTURES

A brief description of the structure is given in this section using the pictorial description in Figure 1 and the

definitions in Table 2. Appendix B contains a more comprehensive discussion of object-oriented concepts

with additional diagrams of the structures. References to the detailed discussion in

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