Encyclopedia of Computer Science and Technology

340 object-oriented programmingPARC laboratory, home of innovative research in graphicaluser interfaces. Smalltalk, like Windows today, treats eachwindow, menu, and other control on the screen as an object(see Smalltalk). Finally, during the 1980s C++ came intoprominence, adding the essential features of object-orientedprogramming to the already very popular C language.Today most popular mainstream languages, including C++,Java, and Visual Basic, are object-oriented (see C++ andJava). Many specialized database languages are also objectoriented.Elements of Object-Oriented ProgrammingThe various object-oriented languages differ somewhat incapabilities, and of course in syntax. However, being objectorientedgenerally implies that the language has the followingfeatures.Classes and ObjectsAn object is defined using a template called a class. A classcontains both the data needed to characterize the objectand the procedures (sometimes called methods or memberfunctions) needed to work with the object (see class).Thus, there could be a class for circles to be drawn on agraphics display. The class might include as its data the ×and y coordinates for the center of the circle, the size of theradius, whether the circle is filled, the color to be used forfilling, and so on. (See C++ for more examples.)When the program needs to use an object of the class,it declares it in the same way it would an ordinary built-indata type such as an integer. Languages such as C++ providefor a special function called a constructor that can beused to define the processing needed when a new object iscreated—for example, memory allocation and setting initialvalues for variables.To access data or functions within a class, the nameof an object of that class is used, followed by a variable orfunction. Thus, if there’s a class called circle, a programmight specify the following:MyCircle Circle; // Declare an object of theCircle classMyCircle.X = 100; // X coordinate on screenMyCircle.Y = 50; // Y coordinate on screenMyCircle.Radius = 25; // Radius in pixelsMyCircle.Filled = True; // A Boolean constantequal to 1MyCircle.FillColor = Blue; // a previouslydefined color constantOnce these specifications have been made, the circlecan be drawn by calling upon its “draw” method or memberfunction:MyCircle.Draw;The designer of a class can choose to restrict access tocertain data items or functions, using a keyword such asprivate or protected. For example, instead of having thepart of the program that uses the class directly set the xand y coordinates, it could keep those variables private andIn the object-oriented C++ programming language data within aclass can be restricted in several ways. Private data can be accessedonly from within the class itself, or from another class declared tobe a “friend” of the containing class. Protected data has these formsof access, plus it can also be accessed from any class derived fromthe containing class. Finally, Public data or functions (methods)can be accessed from anywhere in the program, and provides theinterface by which the class is used.instead provide a method called SetPos. The class mightthen take the coordinates specified by the user and adjustthem to fit the screen dimensions. The Draw method wouldthen use the adjusted internal coordinates rather than thosesupplied originally by the user.InheritanceMany objects are more elaborate or specialized variations ofmore basic objects. For example, in Microsoft Windows thevarious kinds of dialog boxes are specialized versions of thegeneral Window class. Therefore, the specialized versionis created by declaring it to be derived from a “base class.”Put another way, the specialized class inherits the basicdata and functions available in the base (parent) class. Theprogrammer can then add new data or functions or modifythe inherited ones to create the necessary behavior for thespecialized class.Languages such as C++ allow for a class to be derivedfrom more than one base class. This is called multipleinheritance. For example, a Message Window class mightinherit its overall structure from the Window class andits text-display capabilities from the Message class. However,it can sometimes be difficult to keep the relationshipsbetween multiple classes clear. The Java language takes thealternative approach of being limited to only single inheritanceof classes, but allowing interfaces (specifications ofhow a class interacts with the program) to be multiplyinherited.Polymorphism and OverloadingDifferent kinds of objects often have analogous methods.For example, suppose there is a series of classes that representvarious polygons: square, triangle, hexagon, and