Object-oriented analysis and design

Common models used in OOA are the use case and the object model. A use case describes a scenario for standard domain functions that the system must accomplish. An object model describes the names, class relations, operations, and properties of the main objects. User-interface mockups or prototypes can also be created to help understanding. Unlike with OOA that organizes requirements around objects that integrate processes and data, with other analysis methods, processes and data are considered separately. For example, data may be modeled by entity–relationship diagrams, and behaviors by flowcharts or structure charts. Artifacts Outputs of OOA are inputs to OOD. In an iterative approach, and an output artifact does not need to be completely developed to serve as input to OOD. Both OOA and OOD can be performed incrementally, and the artifacts can be continuously grown instead of completely developed in one shot. OOA artifacts include: ; Conceptual model: A conceptual model captures concepts in the problem domain. The conceptual model is explicitly chosen to be independent of implementation details, such as concurrency or data storage. ; Use case: A use case is a description of sequences of events that lead to a system doing something useful. Each use case provides one or more scenarios that convey how the system should interact with the users called actors to achieve a specific business goal or function. Use case actors may be end users or other systems. In many circumstances use cases are further elaborated into use case diagrams. Use case diagrams are used to identify the actor (users or other systems) and the processes they perform. ; System sequence diagram: A system sequence diagram (SSD) is a picture that shows, for a particular scenario of a use case, the events that external actors generate, their order, and possible inter-system events. ; User interface documentation: Optional documentation that shows and describes the look and feel of the user interface. ; Relational data model: If an object database is not used, a relational data model should usually be created before the design since the strategy chosen for object–relational mapping is an output of the OO design process. However, it is possible to develop the relational data model and the OOD artifacts in parallel, and the growth of an artifact can stimulate the refinement of other artifacts. ==Object-oriented design==

OOD, a form of software design, is the process of planning a system of interacting objects to solve a software problem. A designer applies implementation constraints to the conceptual model produced in OOA. Such constraints could include the hardware and software platforms, the performance requirements, persistent storage and transaction, usability of the system, and limitations imposed by budgets and time. Concepts in the analysis model which is technology independent, are mapped onto implementing classes and interfaces resulting in a model of the solution domain, i.e., a detailed description of how the system is to be built on concrete technologies. OOD activities include: • Defining objects and their attributes • Creating class diagrams from conceptual models • Using design patterns • Defining application frameworks • Identifying persistent objects/data • Designing the object relation mapping if a relational database is used • Identifying remote objects OOD principles and strategies include: ; Dependency injection: Dependency injection means if an object depends upon having an instance of some other object, then the needed object is "injected" into the dependent object (for example, being passed a database connection as an argument to the constructor instead of creating one internally). ; Acyclic dependencies principle: The acyclic dependencies principle is that dependency graph of packages or components (the granularity depends on the scope of work for one developer) should have no cycles. This is also referred to as having a directed acyclic graph. For example, package C depends on package B, which depends on package A. If package A depended on package C, you would have a cycle. ; Composite reuse principle: The composite reuse principle is to favor polymorphic composition of objects over inheritance. Artifacts ; Sequence diagram: Extend the sequence diagram to add specific objects that handle the system events. A sequence diagram shows, as parallel vertical lines, different processes or objects that live simultaneously, and, as horizontal arrows, the messages exchanged between them, in the order in which they occur. ; Class diagram: A class diagram is a type of static structure UML diagram that describes the structure of a system by showing the system's classes, its attributes, and the relationships between the classes. The messages and classes identified through the development of the sequence diagrams can serve as input to the automatic generation of the global class diagram of the system. ==See also==