Java™ Application Development on Linux - Dator

3.5 O, Templates! Where Art Thou?93this all out at runtime. That’s polymorphism—Java can deal with these differentobjects while you, the programmer, can describe them in a generalized way.One other related keyword should be mentioned here, abstract. Whenone declares a class as an abstract class, then the class itself is an incompletedefinition. With an abstract class you define all the data of the class but needonly write method declarations, not necessarily all the code for the methods.This makes abstract classes similar to interfaces, but in an abstract class, someof the methods can be fully written out.If you’d like to know more about polymorphism, “late binding,” and moreof this aspect of Java, read Chapter 7 of Eckel’s Thinking in Java. There is anextensive example there with much more detail than we can cover here.3.5O, TEMPLATES! WHERE ART THOU?Programmers familiar with C++ may be wondering how in the world an OOPlanguage without templates can be useful.NOTEActually, something very much like templates is available in Java 5.0. 14 A newfeature, which Sun calls generics, looks an awful lot like C++ templates(including similar syntax). It provides compile-time type checking and implicitcasting when retrieving objects from a generic container.Speaking as programmers who worked with C++ before it had templates,we can sympathize. Java’s previous lack of true templates does impose somelimits on generic programming, but not as much as one might think. Rememberthat unlike C++, all Java classes inherit from exactly one base class, and thatif no base class is specified, they extend the Object class. This means that everysingle Java class either directly or indirectly extends Object, and thus all Javaclasses are instances of Object. So if you need, for example, to implement acontainer, you can guarantee that it can contain any Java class by implementinga container for the Object type. Java also has runtime type identification featuresthat are more than a match for anything C++ has, plus it has type-safe14. Java 5.0 will only be out by the time this book is completed.