Chapter 8

Most of the information on this and other chapters refers directly to material in Wilson, L.B. and Clark, R.G., Comparative Programming Languages (Third Edition, updated by R.G. Clark), Addison-Wesley, September 2000, ISBN 0-201-71012-9.

The material is intended to supplement the textbook by providing further examples and discussion. It is not self-standing, but assumes that you have a copy of the book to hand.

Section 8.3 Inheritance

Supplementary Questions

Java does not support generics. How can a similar effect be simulated in Java. What are the disadvantages of the Java approach compared to that of Ada and C++?
Explain the difference in C++ between an overloaded function and a virtual function. What is a pure virtual function?

Section 8.6 Multiple inheritance

Interfaces

Java applications with a GUI often implement the WindowListener interface. That means that they must provide implementations for seven methods. In very simple situations, six of the implementations do nothing. A simple and useful implementation of the seventh, windowClosing, is to terminate the program when the window is closed. See program listing.
Another commonly used interface in Java is ActionListener. It is described on pages 312-314 and an applet that makes use of it can be run from here.

Section 8.8 Behavioural inheritance

Supplementary Questions

We have a class Person in Java:

class Person {
    private String name;
    public Person(String n) {
        name = n;
    } // constructor
    public String getName() {
        return name;
    } // getName
    public boolean equals(Person p) {
        return name.equals(p.getName())
    } // equals
}

Define a class Student that inherits from Person, but which has an extra String attribute regNum and a method getRegNum. We now want to override the method equals to determine if two objects refer to the same Student, i.e. that they have the same regNum. Why can we not do this? The following declarations and statements may help.

Person p1 = new Student("Jim", "316-2");
Person p2 = new Student("Mary", "417-3");
Person p3 = new Person("Jo");
if (p1.equals(p2)) { ... }
if (p1.equals(p3)) { ... }