CSC 153 Grinnell College Spring, 2005
Computer Science Fundamentals
Laboratory Exercise

Problem Solving in Java, Part II


This laboratory provides additional practice in defining simple Java classes and in designing one class to take advantage of existing classes.

Class Roster Example

Consider the problem of maintaining a roster of students in a class. For each student, we wish to store both an address and a telephone extension. For the roster itself, we wish to support the following operations:

Beginning a Solution

Since the demands of this problem are modest, we look for a relatively simple solution. In particular, we will use the Java class library, making few changes when possible.

A Roster Entry

A student's name could be represented by a Java String. The application consistently uses the student's address and telephone extension as a package, so it seems natural to combine those into a simple AddressPhone class with two fields:

   String address;
   int extension;

In considering needed methods, the application requires creating/initializing an address/extension object and then printing the data. The application does not require changing either field, so we do not need to include any mutator methods. Rather, we provide just two methods:

   // two-parameter constructor:
   AddressPhone (String add, int phone) {
       // initialization code goes here

   String toString() {
       // code to format data within a String goes here

Of course, many additional methods could be defined, but they do not seem needed for this problem

  1. Write class AddressPhone implementing the methods given above.
    Hint: Several elements of AddressPhone can be modeled after the example that we studied in the first Java lab.

  2. Add a main method, so that you can test that the constructor and toString methods work correctly in at least a couple cases.
    Hint: Again from the first Java lab can serve as a model.

A Roster Class

In reviewing the operations needed for this application, we note that the Hashtable class from the previous lab on Hashtables and Inheritance contains almost everything that we need. Thus, our approach will be to take advantage of the Hashtable class, adding a printAll method and a main for testing. Some basic elements of Java object-oriented problem solving and Java make this approach relatively straight forward.

First, we define our new CourseRoster Class, based on the Hashtable Class. This is done with the initial declaration:

   public class CourseRoster extends Hashtable <String, AddressPhone> {
      // CourseRoster will have keys as Strings and values as CoursePhone objects

With this declaration, CourseRoster automatically has all methods implemented in Hashtable. Thus, instead of writing all methods identified at the start of this lab, we only must implement printAll — and perhaps a main for testing. Thus, the full description of CourseRoster has this form:

   public class CourseRoster extends Hashtable <String, AddressPhone> {
      // CourseRoster will have keys as Strings and values as AddressPhone objects

      public void printAll() {
         // In outline:
         //   set up a PrintWriter object for printing
         //   loop through all keys
         //      for each key,
         //          retrieve the corresponding address/phone value
         //          print out the name and address/phone value

      public static void main (String[] args) {
         // declare a couple objectgs:
         CourseRoster csc151 = new CourseRoster();
         CourseRoster csc153 = new CourseRoster();

         // add two students to one course and one to the other
         csc151.put ("Mickey Mouse", new AddressPhone("Hollywood", 314159));
         csc151.put ("Minnie Mouse", new AddressPhone("Hollywood", 271828));
         csc151.put ("Donald Duck",  new AddressPhone("Disneyland", 12345));

         // test required methods

In creating an entry (with the put method, we give an explicit string for the key. For the value, we need an AddressPhone object, with an address and telephone number. Thus, in each case, we create a new object, using our constructor.

  1. Implement class CourseRoster
    Hint: Since the toString method of class AddressPhone already formats the relevant address and telephone information, the print statement can have the form
       out.println ("Student:  " + retrievedKey + "\n" + retrievedValue);

    where retrievedKey and retrievedValue are obtained using methods from the Hashtable class, as indicated in the outline.

A Roster Extension

Suppose that in addition to the operations discussed previously, we also want to maintain a count of the number of students in a class. To accomplish this task, we want to add a field numberStudents within the class and a method classSize.

The definitions of these items is straight forward:

   int numberStudents;

   public int classSize () {
      return numberStudents;

To maintain this count, however, we need to initialize numberStudents in a constructor and then modify the put and remove methods. Strategically, we want these methods to do about what they have done before, but we also want to increment or decrement the numberStudents.

Again, the constructor is easy:

   CourseRoster () {
      numberStudents = 0;

In writing put and remove, we want to change the counter variable, but then use the corresponding operation in Hashtable. Within object-oriented problem solving, we call CourseRoster as subclass of HashTable, since CourseRoster extends HashTable. Similarly, we say HashTable is a superclass of CourseRoster. This jargon explains the needed code in CourseRoster to define the new put method.

   public AddressPhone put (String name, AddressPhone addph) {
      numberStudents++;  // increment the count
      return super.put (name, addph);  // invoke Hashtable's run method 
  1. Implement this expanded Roster class, by adding the variable numberStudents and making adjustments to methods as necessary.

This document is available on the World Wide Web as

created 18 April 2006
last revised 19 April 2006
Valid HTML 4.01! Valid CSS!
For more information, please contact Henry M. Walker at