50 Introduction to (Web design seattle) Computers, the Internet and the
50 Introduction to Computers, the Internet and the Web Chapter 1 Humans learn about objects by studying their attributes and observing their behaviors. Different objects can have similar attributes and can exhibit similar behaviors. Object-oriented design (OOD) models real-world objects. It takes advantage of class relationships, where objects of a certain class such as a class of vehicles have the same characteristics. It takes advantage of inheritance relationships, and even multiple-inheritance relationships, where newly created classes of objects are derived by absorbing characteristics of existing classes and adding unique characteristics of their own. OOD encapsulates data (attributes) and functions (behavior) into objects; the data and functions of an object are intimately tied together. Objects have the property of information hiding. This means that, although objects may know how to communicate with one another across well-defined interfaces, objects normally are not allowed to know how other objects are implemented. Languages such as Java are object-oriented programming in such a language is called object-oriented programming (OOP) and allows designers to implement the object-oriented design as a working system. In Java, the unit of programming is the class from which objects are eventually instantiated (a fancy term for created ). Java classes contain methods (which implement class behaviors) and attributes (which implement class data). Java programmers concentrate on creating their own user-defined types, called classes. Each class contains data and the set of functions that manipulate that data. The data components of a Java class are called attributes. The function components of a Java class are called methods. An instance of a user-defined type (i.e., a class) is called an object. Classes can also have relationships with other classes. These relationships are called associations. With object technology, we can build much of the software we will need by combining standardized, interchangeable parts called classes. The process of analyzing and designing a system from an object-oriented point of view is called object-oriented analysis and design (OOAD). The Unified Modeling Language (the UML) is now the most widely used graphical representation scheme for modeling object-oriented systems. Those who design systems use the language (in the form of graphical diagrams) to model their systems. Over the past decade, the software-engineering industry has made significant progress in the field of design patterns proven architectures for constructing flexible and maintainable object-oriented software. Using design patterns can substantially reduce the complexity of the design process. Design patterns benefit system developers by helping to construct reliable software using proven architectures and accumulated industry expertise, promoting design reuse in future systems, identifying common mistakes and pitfalls that occur when building systems, helping to design systems independently of the language in which they will be implemented, establishing a common design vocabulary among developers and shortening the design phase in a software-development process. Designers use design patterns to construct sets of classes and objects. Creational design patterns describe techniques to instantiate objects (or groups of objects). Structural design patterns allow designers to organize classes and objects into larger structures. Behavioral design patterns assign responsibilities to objects. TERMINOLOGY abstraction ALU (arithmetic and logic unit) Ada ANSI C Copyright 1992 2002 by Deitel & Associates, Inc. All Rights Reserved. 7/8/01
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