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Software Engineering-The Characteristics of OO Methods


Metrics for any engineered product are governed by the unique characteristics of the product. For example, it would be meaningless to compute miles per gallon for an electric automobile. The metric is sound for conventional (i.e., gasoline powered) cars but it does not apply when the mode of propulsion changes radically. Objectoriented software is fundamentally different than software developed using conventional methods. For this reason, the metrics for OO systems must be tuned to the characteristics that distinguish OO from conventional software.

Berard  defines five characteristics that lead to specialized metrics: localization, encapsulation, information hiding, inheritance, and object abstraction techniques. Each of these characteristics is discussed briefly in the sections that follow.

Localization

Localization is a characteristic of software that indicates the manner in which information is concentrated within a program. For example, conventional methods for functional decomposition localize information around functions, which are typically implemented as procedural modules. Data-driven methods localize information around specific data structures. In the OO context, information is concentrated by encapsulating both data and process within the bounds of a class or object.

Because conventional software emphasizes function as a localization mechanism, software metrics have focused on the internal structure or complexity of functions (e.g., module length, cohesion or cyclomatic complexity) or the manner in which functions connect to one another (e.g., module coupling).

Since the class is the basic unit of an OO system, localization is based on objects. Therefore, metrics should apply to the class (object) as a complete entity. In addition, the relationship between operations (functions) and classes is not necessarily one to one. Therefore, metrics that reflect the manner in which classes collaborate must be capable of accommodating one-to-many and many-to-one relationships.

Encapsulation

Berard  defines encapsulation as “the packaging (or binding together) of a collection of items. Low-level examples of encapsulation [for conventional software] include records and arrays, [and] subprograms (e.g., procedures, functions, subroutines, and paragraphs) are mid-level mechanisms for encapsulation.” For OO systems, encapsulation encompasses the responsibilities of a class, including its attributes (and other classes for aggregate objects) and operations, and the states of the class, as defined by specific attribute values.

Encapsulation influences metrics by changing the focus of measurement from a single module to a package of data (attributes) and processing modules (operations). In addition encapsulation encourages measurement at a higher level of abstraction. For example, later in this chapter metrics associated with the number of operations per class will be introduced. Contrast this level of abstraction to conventional metrics that focus on counts of Boolean conditions (cyclomatic complexity) or line of code counts.

Information Hiding

Information hiding suppresses (or hides) the operational details of a program component. Only the information necessary to access the component is provided to those other components that wish to access it.

A well-designed OO system should encourage information hiding. Therefore, metrics that provide an indication of the degree to which hiding has been achieved should provide an indication of the quality of the OO design.

Inheritance

Inheritance is a mechanism that enables the responsibilities of one object to be propagated to other objects. Inheritance occurs throughout all levels of a class hierarchy. In general, conventional software does not support this characteristic.

Because inheritance is a pivotal characteristic in many OO systems, many OO metrics focus on it. Examples (discussed later in this chapter) include number of children (number of immediate instances of a class), number of parents (number of immediate generalizations), and class hierarchy nesting level (depth of a class in an inheritance hierarchy).

Abstraction

Abstraction is a mechanism that enables the designer to focus on the essential details of a program component (either data or process) with little concern for lower-level details. As Berard states: “Abstraction is a relative concept. As we move to higher levels of abstraction we ignore more and more details, i.e., we provide a more general view of a concept or item. As we move to lower levels of abstraction, we introduce more details, i.e., we provide a more specific view of a concept or item.”

Because a class is an abstraction that can be viewed at many different levels of detail and in a number of different ways (e.g., as a list of operations, as a sequence of states, as a series of collaborations), OO metrics represent abstractions in terms of measures of a class (e.g., number of instances per class per application, number or parameterized classes per application, and ratio of parameterized classes to nonparameterized classes).
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