Java™ Application Development on Linux - Dator

86Chapter 3An Experienced Programmer’s Introduction to Java3.4.2 InheritanceInheritance is how a class places itself in a hierarchy of existing classes. In Java,each class inherits from exactly one existing class. A class names the class fromwhich it inherits with the extends keyword. We said a Java class inherits fromexactly one class, and yet our Example 3.24 doesn’t contain the extendskeyword. What gives?If a class doesn’t explicitly extend an existing class, it implicitly extendsthe “root” Java class, Object. The Object class has some interesting features,and the fact that all Java classes directly or indirectly extend Object hasinteresting consequences that we will explore later.Persons coming to Java from another object-oriented language whosename shall remain C++ might wonder about multiple inheritance. Java has theconcept of interfaces. An interface is like a class, except that it may not containdata 10 and may contain only method definitions, without any implementation(Example 3.25). An interface may not be instantiated (created with the newoperator), 11 so how do you make use of interfaces? Well, a class extends exactlyone existing base class, but it may implement any number of interfaces by usingthe implements keyword (Example 3.26).Example 3.25 An interfacepublic interfaceIdentifiable{public int getID();}As you can see, a class that implements an interface must provide an implementationof all the methods defined in the interface. We said that an interfacecannot be instantiated, but you can declare a variable of type Identifiable10. Actually, an interface may contain final static data, but since we haven’t introducedthese concepts yet, just pretend interfaces cannot contain data for now and we’ll put the lie toit later.11. Although you can do something that looks suspiciously like it with anonymous innerclasses—but since we haven’t introduced these concepts yet, just pretend that you cannot instantiatean interface; you will see such use later.