Computer Science 15-100, Fall 2008
Class Notes:  Getting Started with Writing Classes

1. Mark's Fraction Code
2. David's Fraction Code

Getting Started with Writing Classes

1. Mark's Fraction Code

   // Mark's Foo Code (tests the Fraction class)
   // This code was developed in class.  As such, it is
   // for demonstrational purposes only, and in particular
   // may be incomplete and may contain bugs or other oddities.
   class Foo
   {
     public static void main(String[] args)
     {
       Fraction f1 = new Fraction(1,2);
       System.out.println("f1 = " + f1.toString());
       Fraction f2 = new Fraction(0,4);
       Fraction f3 = new Fraction(8);
       Fraction f4 = new Fraction();
       System.out.println("f2 = " + f2);
       System.out.println(f1.add(f2));
       System.out.println(f3 + " " + f4);
     }
   }
   
   // Mark's Fraction Code
   // This code was developed in class.  As such, it is
   // for demonstrational purposes only, and in particular
   // may be incomplete and may contain bugs or other oddities.
   class Fraction
   {
     private int numerator;
     private int denominator;
   
     public Fraction(int numerator, int denominator)
     {
       int commonFactor = gcd(numerator, denominator);
       this.numerator = numerator/commonFactor;
       this.denominator = denominator/commonFactor;
     }
   
     public Fraction(int num)
     {
       numerator = num;
       denominator = 1;
     }
   
     public Fraction()
     {
       this(0,1);
     }
   
     public Fraction add(Fraction other)
     {
       int num = this.numerator * other.denominator +
                 other.numerator * this.denominator;
       int denom = this.denominator * other.denominator;
       return new Fraction(num, denom);
     }
   
     public boolean equals(Fraction other)
     {
       return (this.numerator == other.numerator &&
               this.denominator == other.denominator);
     }
   
     public String toString()
     {
       return numerator + "/" + denominator;
     }
   
     private static int gcd(int x, int y)
     {
       int r;
       while (y > 0)
       {
         r = x % y;
         x = y;
         y = r;
       }
       return x;
     }
   }

2. David's Fraction Code

   // David's FractionDemo Code
   // This code was developed in class.  As such, it is
   // for demonstrational purposes only, and in particular
   // may be incomplete and may contain bugs or other oddities.
   class FractionDemo {
     public static void main(String[] args) {
       Fraction x = new Fraction(4,6);
       System.out.println(x.toString());
       System.out.println(x); // implicitly calls toString
       Fraction y = new Fraction(1,6);
       System.out.println(y);
   
       Fraction z = x.times(y); // like String
       System.out.println(x + " * " + y + " = " + z);
       
       x = new Fraction(2,3);
       y = new Fraction(4,3);
       z = x.plus(y);
       System.out.println(x + " + " + y + " = " + z);
       
       Fraction z2 = new Fraction(4,2);
       System.out.println(z + " equals " + z2 + " is " +
                          z.equals(z2));
   
       z2 = new Fraction(-4,-2);
       System.out.println(z + " equals " + z2 + " is " +
                          z.equals(z2));
   
       x = new Fraction(0,9);
       System.out.println(x);
       x = new Fraction(9,0);
       System.out.println(x);
       x = new Fraction();
       System.out.println(x);
     }
   }
   
   // David's Fraction Code
   // This code was developed in class.  As such, it is
   // for demonstrational purposes only, and in particular
   // may be incomplete and may contain bugs or other oddities.
   class Fraction {
     private int num, den;
     
     // Constructors
     public Fraction() {
       this.num = 0;
       this.den = 1;
     }
     
     public Fraction(int num, int den) {
       // handle the sign -- only the num can be negative
       if (den < 0) {
         den = -den;
         num = -num;
       }
       // and assign to the instance variables
       this.num = num;
       this.den = den;
       // reduce them
       int gcd = gcd(num, den);
       if (gcd > 0) {
         this.num /= gcd;
         this.den /= gcd;
       }
     }
     
     private static int gcd(int x, int y) {
       // Euclid:  gcd(x, y) == gcd(y, x%y)
       // gcd(180, 24) = gcd(24, 12)
       //              = gcd(12, 0)
       // gcd(24, 180) = gcd(180, 24)..
       x = Math.abs(x);
       y = Math.abs(y);
       if ((x == 0) || (y == 0)) return 0;
       while (y != 0) {
         int r = x % y;
         x = y;
         y = r;
       }
       return x;
     }
   
     // a/b * c/d = ac/bd
     public Fraction times(Fraction that) {
       int num = this.num * that.num;
       int den = this.den * that.den;
       return new Fraction(num, den);
     }
   
     // a/b + c/d
     // = a/b * d/d  + c/d * b/b
     // = ad/bd + bc/bd
     // = (ad + bc)/bd
     public Fraction plus(Fraction that) {
       int num = this.num*that.den + that.num*this.den;
       int den = this.den*that.den;
       return new Fraction(num, den);
     }
     
     public String toString() {
       if (den == 0)
         return "NaF"; // Not A Fraction
       else if (num == 0)
         return "0";
       else if (den == 1)
         return ("" + num);
       else
         return num + "/" + den;
     }
     
     public boolean equals(Fraction that) {
       return ((this.num == that.num) &&
               (this.den == that.den));
     }
   }
   

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