Programming for the Web: Homework 2

Writing a Java application with class structure

There are two types of Java programs: applications and applets. An applet is a web-based Java program embedded in an HTML document. An application, on the other hand, is not web-based. It is developed and run at the command line, much like a C program.

Your first Java programming assignment is to write a small application. Before starting this assignment, we assume that you are familiar with any Java programming environment. You may
use Java compilers on different systems, or a programming environment such as J++ or Cafe'.

Writing the programs

Create a class called Rational for performing arithmetic with fractions, and write a test program to use this class: (This is based on assignment 6.3 in the Deitels book.)

Use integer variables to represent private instance variables of the class - the numerator and the denominator. The fractions are always to be stored in lowest terms (i.e. if you get or compute 2/4, store 1/2 instead). You can do this by writing a reduce method as described below, which all the other methods can call.

Write one constructor function that that initializes an object with an initial numerator and denominator (and reduces to lowest terms). Write another constructor function that takes no arguments and initializes the number to some default value. 0 would be a good choice; note that 0 is stored as the rational number 0/1.

You should also write the following public methods. Since this is supposed to be an exercise in writing Java classes and not in remembering 6th grade arithmetic, all the definitions are given for you.

Addition of two Rational numbers. The result of this and all the remaining methods should be in lowest terms.

 a/b + c/d = (a*d + c*b) / b*d

Subtraction of two Rational numbers.

 a/b - c/d = (a*d - c*b) / b*d

Multiplication of two Rational numbers.

 a/b * c/d = a*c / b*d

Comparison of two Rational numbers.

 a/b < c/d iff (a*d < c*b)

A method called toString which converts a Rational number with numerator a and denominator b to "a/b", i.e. the "/" is a character in the string.

It will be convenient to also write a private method reduce which will always reduce the number represented by numerator u and denominator v to lowest terms. In most cases, you find the lowest terms by dividing both numbers by the greatest common divisor (gcd). An algorithm to find the gcd of two positive numbers is sketched below:

        // assume u > 0, v > 0
        private int gcf (int u, int v)
        {  int t;
           while (u > 0)
           {
             if (u < v) { t=u; u=v; v=t; }
             u = u - v;
           }
           return v;
        }

You must also write a test program for this class. Write an application that reads a set of numerators and denominators from standard input, converting these to Rational numbers and storing them in an array (i.e. array elements are of type Rational). Now find the maximum number in the array and sum the array elements.

What to put on the homework page as the submission of your homework

Submit your assignment by linking the description of your program, a link to the Java source code and a small file showing sample output run (like my .run files) to your homework page.

One way to make the .run file if you're running on carver, or another Unix system, is to type "script MyProgram.run" at the command prompt. Then everything that is typed on your workstation is copied into this file. So run your program "java MyProgram" and when you're done, type ^D (control-D) at the command line to finish the script. Then the file MyProgram.run will show how your program runs.

For how to write a description of your homework and document your program, and other information, please see these homework guidelines.