Maintainability

Introduction

The Maintainability Index

Metrics for Object-oriented Programs

• WMC: Weighted methods per class
• DIT: Depth of Inheritance Tree
• NOC: Number of Children
• CBO: Coupling between object classes
• RFC: Response for a Class
• LCOM: Lack of cohesion in methods

Dependency Metrics on Packages

• Efferent (outward) couplings measure the number of packages the package we are examining depends on.
• Afferent (inward) couplings measure the number of other packages that depend on the package we are examining.
• Stable Dependencies Principle dependencies in our packages should follow the direction of stability: less stable packages should depend on more stable ones.

A stable package in the Eclipse distribution

Analyzability

• Consistency
• Expression Formatting
• Statement Formatting
• Naming Conventions
• Statement-level Comments
• Versioning Comments
• Visual Structure: Blocks and Indentation
• Length of Expressions, Functions, and Methods
• Control Structures
• Boolean Expressions
• Recognizability and Cohesion
• Dependencies and Coupling (later)
• Code Block Comments
• Data Declaration Comments
• Appropriate Identifier Names
• Locality of Dependencies
• Ambiguity
• Reviewability

Analyzability: Dependencies and Coupling

• Data Coupling: data passed to a routine
• Stamp Coupling: data passed to a routine, but only small part is used
• Control Coupling: data affects control decisions
• Temporal Coupling: behavior depends on the order elements are called (also lack of reentrancy)
• Common Coupling: sharing of data through global variables
• External Coupling: implicit sharing of knowledge of data formats or protocols
• Content Coupling: one module depends or modifies internal workings of another

Changeability

• Identification
• Separation

Stability

• Encapsulation and Data Hiding
• Data Abstraction
• Type Checking
• Compile-time Assertions
• Run-time Checks and Inspection-time Assertions

Testability

• Unit Testing
• Integration Testing
• System Testing
• Test Coverage Analysis
• Incidental Testing

Effects of the Development Environment

• Incremental Builds
• Tuning Build Performance

Further Reading

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Exercises and Discussion Topics

• How can a metric like the maintainability index be used profitably within the software development process, without creating the wrong incentives for programmers?
• Go through the project you've been working on and tighten the access control so that all elements (classes, class members, functions) are declared with the most conservative visibility.
• Write a test harness to test your system's random number generator. How ``random'' are the numbers it provides?
• Download the ckjm (http://www.spinellis.gr/sw/ckjm/) (Chidamber and Kemerer Java Metrics) metrics tool, and apply it to code in your environment. Select some classes that would be flagged using the criteria we described and explain whether there is indeed a problem with their design, and, if there is, how you propose to improve it.

• Contents

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