CSC152 2006S, Class 21: Standard Object Methods

Admin:
* Trustees voted to split Math/CS into CS and Math/Stats.  Yay!
* Should [some student] be allowed to leave early?  No one cares, but the general consenus is "I hope you fail".
* Two readings for tomorrow (both ready already):
  + Interfaces
  + Polymorphism
* Questions on the exam?
* Office hours today: 2-4 p.m.

Overview:
* The standard methods.
* Lab.

/Questions/

* What is a zero-parameter constructor?
  One with no parameters
* What's the difference between a constructor and a method?
  * A method can do anything.
  * Constructors are intended to initialize a new object in a class.
  * Methods are defined with
        public TYPE NAME(PARAMS)
  * Constructors are defined
        public CLASS(PARAMS)
  * What should a zero-parameter constructor do?
    * Initialize all the fields to reasonable values
    * In a piggybank, set all the counts to 0.
    * In a fraction, set the numerator to 0 and the denominator to 1.
  * How do I round a double value?  (Some different strategies)
    * Convert it to a Double and then call intValue
    * Try (int) nameOfDouble
    * Convert it to a string, strip off the period, and then call Integer.parseInt
    * Use Math.round or Math.floor or something similar
    * E.g.,
        this.numerator = BigInteger.valueOf(Math.round(Math.floordecimal));
        this.denominator = BigInteger.ONE;
  * Do you care that we do lots of conversions?
    * If they're necessary, no.  If they're just for the sake of converting, then yes.
  * How close is close enough when deciding if a fraction is good enough?
    if (larger.subtract(smaller).toDecimal < .000000001)
  * How do I use Math.PI?
    Fraction pie = new Fraction(Math.PI);
    pen.println(pi + " is approximately " + pie);
    pen.println(pie + " is approxmately " + pie.toDouble());

/Standard Methods/

* Java programmers assume that whenever you define a class, you define some methods
  * Always: 
    toString() - get a pritnable version of the object
    equals(Object other) - compare
    hashCode() - compute some integer that is likely to be different for different objects and must be the same for equal objects
  * Usually
     compareTo(XXX other)
     clone() - Make a copy
* When writing equals, the standard form is the following.  Only "MyClass" and the ellipses need to be changed

        public boolean equals(Object other)
        {
          return (other instanceof MyClass) &&
                 this.equals((MyClass) other);
        }

        public boolean equals(MyClass other)
        {
          ...
        }

/Questions from the Lab/

* How do I determine whether two fractions are equal when they're both not in simplified form?
  * Eryn says "cross product"