Concepts of Programming Languages

12.7 Support for Object-Oriented Programming in Java 555

implementation of a method to compare the elements to be sorted. The

generic Comparable interface provides the protocol for this comparing

method, which is named compareTo. The code for the Comparable inter-

face is as follows:

public interface Comparable <T> {

public int compareTo(T b);

}

The compareTo method must return a negative integer if the object

through which it is called belongs before the parameter object, zero if they are

equal, and a positive integer if the parameter belongs before the object through

which compareTo was called. A class that implements the Comparable inter-

face can sort the contents of any array of objects of the generic type, as long as

the implemented compareTo method for the generic type is implemented and

provides the appropriate value.

In addition to interfaces, Java also supports abstract classes, similar to

those of C++. The abstract methods of a Java abstract class are represented as

just the method’s header, which includes the abstract reserved word. The

abstract class is also marked abstract. Of course, abstract classes cannot be

instantiated.

Chapter 14 illustrates the use of interfaces in Java event handling.

12.7.3 Dynamic Binding

In C++, a method must be defined as virtual to allow dynamic binding. In Java,

all method calls are dynamically bound unless the called method has been

defined as final, in which case it cannot be overridden and all bindings are

static. Static binding is also used if the method is static or private, both of

which disallow overriding.

12.7.4 Nested Classes

Java has several varieties of nested classes, all of which have the advantage of

being hidden from all classes in their package, except for the nesting class. Non-

static classes that are nested directly in another class are called inner classes.

Each instance of an inner class must have an implicit pointer to the instance

of its nesting class to which it belongs. This gives the methods of the nested

class access to all of the members of the nesting class, including the private

members. Static nested classes do not have this pointer, so they cannot access

members of the nesting class. Therefore, static nested classes in Java are like

the nested classes of C++.

Though it seems odd in a static-scoped language, the members of the

inner class, even the private members, are accessible in the outer class. Such

references must include the variable that references the inner class object. For