In the past, information systems used to be defined primarily by their functionality: data and functions were kept separate and linked together by means of input and output relations.
The object-oriented approach, however, focuses on objects that represent abstract or concrete things of the real world. These objects are first defined by their character and their properties which are represented by their internal structure and their attributes (data). The behaviour of these objects is described by methods (functionality).
Objects form a capsule which combines the character to the respective behaviour. Objects should enable programmers to map a real problem and its proposed software solution on a one-to-one basis.
Typical objects in a business environment are, for example, ‘Customer’, ‘Order’, or ‘Invoice’. From Release 3.1 onwards, the Business Object Repository (BOR) of SAP Web Applicaton Server ABAP has contained examples of such objects. The BOR object model will be integrated into ABAP Objects in the next Release by migrating the BOR object types to the ABAP class library.
A comprehensive introduction to object orientation as a whole would go far beyond the limits of this introduction to ABAP Objects. This documentation introduces a selection of terms that are used universally in object orientation and also occur in ABAP Objects. In subsequent sections, it goes on to discuss in more detail how these terms are used in ABAP Objects. The end of this section contains a list of further reading, with a selection of titles about object orientation.
Objects are instances of classes. They contain data and provides services. The data forms the attributes of the object. The services are known as methods (also known as operations or functions). Typically, methods operate on private data (the attributes, or state of the object), which is only visible to the methods of the object. Thus the attributes of an object cannot be changed directly by the user, but only by the methods of the object. This guarantees the internal consistency of the object.
Classes describe objects. From a technical point of view, objects are runtime instances of a class. In theory, you can create any number of objects based on a single class. Each instance (object) of a class has a unique identity and its own set of values for its attributes.
In a program, you identify and address objects using unique object references. Object references allow you to access the attributes and methods of an object.
In object-oriented programming, objects usually have the following properties:
Objects restrict the visibility of their resources (attributes and methods) to other users. Every object has an interface, which determines how other objects can interact with it. The implementation of the object is encapsulated, that is, invisible outside the object itself.
You can use an existing class to derive a new class. Derived classes inherit the data and methods of the superclass. However, they can overwrite existing methods, and also add new ones.
Identical (identically-named) methods behave differently in different classes. In ABAP Objects, polymorphism is implemented by redefining methods during inheritance and by using constructs called interfaces.
Below are some of the advantages of object-oriented programming:
· Complex software systems become easier to understand, since object-oriented structuring provides a closer representation of reality than other programming techniques.
· In a well-designed object-oriented system, it should be possible to implement changes at class level, without having to make alterations at other points in the system. This reduces the overall amount of maintenance required.
· Through polymorphism and inheritance, object-oriented programming allows you to reuse individual components.
· In an object-oriented system, the amount of work involved in revising and maintaining the system is reduced, since many problems can be detected and corrected in the design phase.
Achieving these goals requires:
· Object-oriented programming languages
Object-oriented programming techniques do not necessarily depend on object-oriented programming languages. However, the efficiency of object-oriented programming depends directly on how object-oriented language techniques are implemented in the system kernel.
· Object-oriented tools
Object-oriented tools allow you to create object-oriented programs in object-oriented languages. They allow you to model and store development objects and the relationships between them.
· Object-oriented modeling
The object-orientation modeling of a software system is the most important, most time-consuming, and most difficult requirement for attaining the above goals. Object-oriented design involves more than just object-oriented programming, and provides logical advantages that are independent of the actual implementation.
This section of the ABAP User’s Guide provides an overview of the object-oriented extension of the ABAP language. We have used simple examples to demonstrate how to use the new features. However, these are not intended to be a model for object-oriented design. More detailed information about each of the ABAP Objects statements is contained in the keyword documentation in the ABAP Editor. For a comprehensive introduction to object-oriented software development, you should read one or more of the titles listed below.
There are many books about object orientation, object-oriented programming languages, object-oriented analysis and design, project management for OO projects, patterns and frameworks, and so on. This is a small selection of good books covering the most important topics:
· Scott Ambler, The Object Primer, SIGS Books & Multimedia (1996), ISBN: 1884842178
A very good introduction to object orientation for programmers. It provides comprehensive explanations of all essential OO concepts, and contains a procedure model for learning OO quickly and thoroughly. It is easy to read and practical, but still theoretically-founded.
· Grady Booch, Object Solutions: Managing the Object-Oriented Project, Addison-Wesley Pub Co (1995), ISBN: 0805305947
A good book about all of the non-technical aspects of OO that are equally important for effective object-oriented programming. Easy to read and full of practical tips.
· Martin Fowler, UML Distilled: Applying the Standard Object Modeling Language, Addison-Wesley Pub Co (1997), ISBN: 0201325632
An excellent book about UML (Unified Modeling Language - the new standardized OO language and notation for modeling). Assumes knowledge and experience of object orientation.
· Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, Design Patterns. Elements of Reusable Object-Oriented Software, Addison-Wesley Pub Co (1998), ISBN: 0201634988
Provides a pattern, showing how recurring design problems can be solved using objects. This is the first big pattern book, containing many examples of good OO design.
· James Rumbaugh, OMT Insights: Perspectives on Modeling from the Journal of Object-Oriented Programming, Prentice Hall (1996), ISBN: 0138469652
A collection of articles addressing the many questions and problems of OO analysis and design, implementation, dependency management, and so on. Highly recommended.
If you are new to object-orientation, you should read Scott Ambler’s ‘The Object Primer’ and then acquire some practical experience of your own. You should definitely use the CRC techniques described by Ambler and Fowler for object-oriented analysis and design. After this, you should learn UML, since this is the universal OO analysis and design notation. Finally, you should read at least one book about patterns.
At the beginning of a large OO project, the question immediately arises as to the sequence in which things should be done, which phases should be finished at what time, how to divide up and organize the development work, how to minimize risks, how to assemble a good team, and so on and so forth. Many of the best practices of project management have had to be redefined for the object-oriented world, and the opportunities that this has produced are significant. For further information about how to use OO in project management, see Grady Brooch’s book ‘Object solutions’, or the chapter entitles ‘An outline development process’ from Martin Fowler’s book.
There are, of course, many other good books about object orientation. The above list does not claim either to be complete, or necessarily to recommend the best books available.