Computer Science 15-100, Spring 2009
Class Notes:  Getting Started with Classes

This week we did not follow a formal outline.  Instead, we motivated the idea behind classes, objects, and object-oriented programming by interactively designing the following two classes:  Point and Fraction.

// The Point class
// Today in class we evolved code from a traditional non-object-oriented approach to an object-oriented approach.  Here is the code as it stood by the end of class:

// Demonstrates the use of our new "Point" class.
class ClassDemo {
  public static void main(String[] args) {
    Point point = new Point(5,7); // create a new Point instance (object)
    System.out.println(point);    // print it out, using its toString method

    int dx = 3;
    int dy = -8;

    point.move(dx, dy);           // change (mutate) it, using its move method
    System.out.println(point);    // and print it out again
  }
}

class Point {
  // properties (fields)
  private int x, y;
  
  // constructor (initializer)
  // public void Constructor(...)
  public Point(int initialX, int initialY) {
    x = initialX;
    y = initialY;
  }
  
  // accessors ("getters")
  public int getX() { return x; }
  public int getY() { return y; }
  
  // mutators ("setters")
  public void setX(int newX) { x = newX; }
  public void setY(int newY) { y = newY; }

  public void move(int dx, int dy) {
    setX(x + dx);
    setY(y + dy);
  }

  // toString
  public String toString() {
    return "(" + x + "," + y + ")";
  }
}

// The Fraction class
// Today in class we continued to explore creating our own class.  Here is the code as it stood by the end of class:  

class FractionDemo {
  public static void main(String[] args) {
    Fraction f1 = new Fraction(1,2); // 1/2    
    Fraction f2 = new Fraction(2,6); // 1/3    
    Fraction f3 = f1.times(f2);        // wanted: f3 = f1 * f2;
    System.out.println(f1 + " * " + f2 + " = " + f3);

    Fraction f4 = new Fraction(1,6); // 1/6
    System.out.println(f4);

    assert(f3.equals(f4));           // wanted: f3 == f4
  }
}

class Fraction {
  private int num, den; // numerator and denominator
  
  // a/b * c/d == (ac)/(bd)
  public Fraction times(Fraction that) {
    return new Fraction(this.num * that.num, this.den * that.den);
  }
  
  // public void Constructor(int n, int d)
  public Fraction(int n, int d) {
    int gcd = gcd(n, d);    
    num = n/gcd;
    den = d/gcd;
  }

  // @TODO: does not work for negatives, maybe not for 0...
  private int gcd(int x, int y) {
    if ((x == 0) || (y == 0)) return 1;
    if (x < 0) x = -x;
    if (y < 0) y = -y;
    while (y > 0) {
      int r = x % y;
      x = y;
      y = r;
    }
    return x;
  }
  
  public String toString() {
    return num + "/" + den;
  }
  
  public boolean equals(Fraction that) {
    return ((this.num == that.num) && (this.den == that.den));
  }
}

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