Computer Science 15-100 (Sections T & U), Spring 2008
Class Notes:  Writing Classes (Day 2)

Logistics

1. Schedule
   1. Due today:  hw8b extension, hw9a, bonus3
   2. quiz7 today
   3. Due Friday:  hw9b
   4. Bonus Lecture #4 (Tetris!) is today after class
2. Reading:
   1. Ch 4:  Writing Classes

Topic Outline:

Varargs:  Variable-length arguments as arrays

  public static void main(String[] args) throws Exception {
    System.out.println(max(1,3,5,7,6,4,2));
  }
  
  public static int max(int... a) {
    if (a.length == 0) return 0;
    int max = a[0];
    for (int i=1; i<a.length; i++) max = Math.max(max,a[i]);
    return max;
  }

We will start with our simple Fraction class (from last lecture)

(Note that the new state of our code from the end of class, demonstrating the concepts from today's lecture, is appended below.)

1. Visibility Modifiers
   1. public:  visible to everyone
   2. protected:  only visible within the given package or within subclasses
   3. default:  only visible within the given package
   4. private:  only visible within the given class
       
2. Instance Variables
   1. Non-static
   2. Require some object reference to access (hence, "instance" variables)
   3. Should be private (maybe default?)
   4. Can be initialized outside of a constructor
      1. Initialized before constructor is called
   5. Scope
      1. Visible from all instance methods (but not static methods!)
      2. Shadowing
      3. Using this.foo to escape shadowing
          
3. Accessors and Mutators
   1. Accessor:  getFoo
   2. Mutator:   setFoo, clearFoo, incrementFoo, etc...
   3. For booleans:  isProperty, setProperty
       
4. Constructors
   1. Default Constructor:  No args, get this for free unless you implement a constructor
   2. Invoking "super" constructor
   3. Invoking "this" constructor
       
5. The toString method
   1. Every class gets a default toString method
   2. Can "override" this with your own
      1. Must have exactly this signature, visibility, and return type:
         public String toString()
          
6. A Class's API (Application Programming Interface)
   1. All your public methods, and only your public methods
   2. What methods should be public in the Rational class?
       
7. Encapsulation, Data Hiding, Self-Governing:
         Control access to, and changing of, object properties
   which is roughly equivalent to:
         Expose public interface without reference to private implementation

Here is our code at the end of class:

public class FractionExample {

    public static java.util.Scanner scanner = new java.util.Scanner(System.in);
    public static java.util.Random random = new java.util.Random();

    public static void main(String[] args) {
      Fraction x = new Fraction(2,7);
      Fraction y = new Fraction(3,7);
      System.out.println("x = " + x);
      System.out.println("y = " + y);
      Fraction z = x.times(y);
      System.out.println("z = " + z);
      System.out.println("z's numerator is:  " + z.getNum());
      z.setNum(z.getNum() + 1);
      System.out.println("z = " + z);
    }    
}

class Fraction {
  // instance properties
  // instance == object
  // properties == variables == fields == attributes
  private int num = 10, den;
  
  // accessors
  public int getNum() {
    return this.num;
  }
  
  public int getDen() {
    return this.den;
  }
  
  // mutators
  public void setNum(int num) {
    this.num = num;
    reduce();
  }

  public void setDen(int den) {
    this.den = den;
    reduce();
  }

  private static int gcd(int i1, int i2) {
    int gcd = 1;
    for (int i=2; i<=Math.min(i1,i2); i++)
      if ((i1 % i == 0) && (i2 % i == 0)) gcd = i;
    return gcd;
  }
  
  public Fraction(int num, int den) {
    // System.out.println("make a fraction with n = " + n + " and d = " + d);
    this.num = num;
    this.den = den;
    reduce();
  }
  
  private void reduce() {
    int gcd = gcd(this.num,this.den);
    this.num = this.num / gcd;
    this.den = this.den / gcd;    
  }

  //  this    that
  // (a/b) + (c/d) == ((ad + bc) / (bd))
  public Fraction plus(Fraction that) {
    int n = ((this.num * that.den) + (this.den * that.num));
    int d = (this.den * that.den);
    return new Fraction(n,d);
  }

  public Fraction times(Fraction that) {
    int n = this.num * that.num;
    int d = this.den * that.den;
    return new Fraction(n,d);
  }
  
  public Fraction pow(int p) {
    int n = 1;
    int d = 1;
    for (int power=1; power<=p; power++) {
      n *= this.num;
      d *= this.den;
    }
    return new Fraction(n,d);
  }

  public String toString() {
    return "" + num + "/" + den;
  }
}

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