Software II Concepts

Subsystems Guidline 1

Minimize Number of Collaborations

Subsystems Guidline 2

Minimize Delegations of Subsystem Contracts

Subsystems Guidline 3

Minimize Number of Contracts

Cohesion

The degree to which all elements of a component are directed towards a single task.

High cohesion is good because if there is a change, there is a high probability that the impact will be localized.

Coincidental Cohesion

Parts of the component are unrelated.

Logical Cohesion

Elements of a component are related logically but not functionally.

Temporal Cohesion

Elements are related by timing involved.

Procedural Cohesion

Related only to ensure a particular order of execution.

Communicational Cohesion

Functions performed on the same data or to produce the same data.

Sequential Cohesion

The output of one part is the input to another.

Functional Cohesion

Every essential element to a single computation is contained in this component.

Exceptional Post Condition

Says what is true when a method throws an exception.

Process for Writing Protocols

For each class, For each contract, For each responsibility - Specify complete protocol (set of signatures) to

support the responsibility.

A Protocol

Set of signatures for methods to be implemented.

Coupling

The degree of dependence such as the amount of interactions among components.

Changes to a component are localized and do not cascade.

Content Coupling

One component modifies another.

Common Coupling

More than one component share data such as global data structures.

Problem: Difficult to determine components that edit.

External Coupling

Two components share something externally imposed.

Low = changes in external entity has little impact

Medium = Some impact (introduce interfaces)

High = Substantial impact

Control Coupling

Component passes control parameters (boolean flags) to coupled components.

Bad if control flow paths are unrelated, good if reusable.

Stamp Coupling

Component passes a data structure to another component that does not need access to the entire structure.

Problem: Requires 2nd component to know how to manipulate data, Security Risks.

Data Coupling

Component passes data (NOT DATA STRUCTURES) to other component.

Uncoupled

Completely uncoupled components are not systems.

Semi-formal Notation

Private Responsibilities

Responsibilities not part of a contract.