Please read through the following so we have a common basis for Thurs night, rather than hearing it for the first time when I present it. I'd like to brainstorm the basic design of a theoretical invoicing application in light of these ideas. Adapted from Wikipedia: Intro: Object-oriented design is part of OO methodology and it forces programmers to think in terms of objects, rather than procedures, when they plan their code. An object contains encapsulated data and procedures grouped together to represent an entity. The 'object interface', how the object can be interacted, is also defined. An object-oriented program is described by the interaction of these objects. Object-oriented design is the discipline of defining the objects and their interactions to solve a problem that was identified and documented during object-oriented analysis. From a business perspective, Object Oriented Design refers to the objects that make up that business. For example, in a certain company, a business object can consist of people, data files and database tables, artifacts, equipment, vehicles, etc. Sources for object-oriented design: <---- Very important!!!! The input for object-oriented design is provided by object-oriented analysis. Analysis and design may occur in parallel, and in practice the results one activity can feed the other in a short feedback cycle through an iterative process. Both analysis and design can be performed little by little, and the artifacts can be continuously grown instead of completely developed in one shot. Some typical sources for object-oriented design are: Conceptual model: Conceptual model is the result of object-oriented analysis, it 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: Use case is description of sequences of events that, taken together, 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 use to identify the actor (users or other systems) and the processes they perform. User interface documentations (if applicable): Document that shows and describes the look and feel of the end product's user interface. It is not mandatory to have this, but it helps to visualize the end-product and therefore helps the designer. Relational data model (if applicable): A data model is an abstract model that describes how data is represented and used. If an object database is not used, the 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 object-oriented design artifacts in parallel, and the growth of an artifact can stimulate the refinement of other artifacts. Object-oriented concepts supported by an OO language The five basic concepts of object-oriented design are the implementation level features that are built into the programming language. These features are often referred to by these common names: 1. Object/Class: A tight coupling or association of data structures with the methods or functions that act on the data. This is called a class, or object (an object is created based on a class). Each object serves a separate function. It is defined by its properties, what it is and what it can do. An object can be part of a class, which is a set of objects that are similar. 2. Information hiding: The ability to protect some components of the object from external entities. This is realized by language keywords to enable a variable to be declared as private or protected to the owning class. <-- Buggy in VFP 3. Inheritance: The ability for a class to extend or override functionality of another class. The so-called subclass has a whole section that is the superclass and then it has its own set of functions and data. 4. Interface: <-- Weak in VFP The ability to defer the implementation of a method. The ability to define the functions or methods signatures without implementing them. Polymorphism: The ability to replace an object with its subobjects. The ability of an object-variable to contain, not only that object, but also all of its subobjects. Designing concepts Defining objects, creating class diagram from conceptual diagram: Usually map entity to class. Identifying attributes. 1. Use design patterns A design pattern is not a finished design, it is a description of a solution to a common problem. The main advantage of using a design pattern is that it can be reused in multiple applications. It can also be thought of as a template for how to solve a problem that can be used in many different situations and/or applications. Object-oriented design patterns typically show relationships and interactions between classes or objects, without specifying the final application classes or objects that are involved. 2. Define application framework Application framework is a term usually used to refer to a set of libraries or classes that are used to implement the standard structure of an application for a specific operating system. By bundling a large amount of reusable code into a framework, much time is saved for the developer, since he/she is saved the task of rewriting large amounts of standard code for each new application that is developed. 3. Identify persistent objects/data (if applicable): Identify objects that have to last longer than a single runtime of the application. If a relational database is used, design the object relation mapping. <-- In VFP Objects can exist momentarily (like a function call) or last as long as the app is running. A single createobject() may be the only time/duration for which an object is used. Identify and define remote objects (if applicable). Some design principles and strategies Dependency injection: The basic idea is that 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. <- We don't see this in VFP code, but very useful. Acyclic dependencies principle: <- I don't know what this even means in VFP or in any other language. Favor polymorphic composition of objects over inheritance. References: The GOF book: Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley. 1995. ISBN 0-201-63361-2. To unsubscribe from the NYACC Database SIG list, send an email to nycfoxsig-request@xxxxxxxxxxxxx with 'unsubscribe' (no quotes) in the subject field.