CS 61B  Homework 2
                       Due 5pm Friday, February 6, 2004

This homework assignment is designed to help you learn about building Java
classes and to observe the decomposition of a complicated task into simple
subtasks.  This is an individual assignment; you may not share code with other
students.

Read Chapters 4-6 of Arnow/Dexter/Weiss.  Copy the Homework 2 directory by
doing the following, starting from your home directory:
	mkdir hw2
	cd hw2
	cp $master/hw/hw2/* .

Your task is to fill in the implementation of a class that manipulates dates.
Do not use any of the built-in operations on dates provided in the Java library
in your solution.  The overall task is broken down into subtasks, which we
suggest you implement in a bottom-up order, so that you can easily test as you
go.  The grading test cases give partial credit for the more basic operations,
even if some of the higher level operations do not work properly.

Please observe these notes on grading.

1)  Your program must compile without errors to receive any credit on this
    assignment.  If only one or two of your methods work, remove any code that
    causes problems for "javac" before submitting your solution.  However,
    don't remove any of the method declarations that appear in the skeletal
    Date.java we give you.
2)  We have provided a main method in the Date class that tests some of your
    methods.  You are welcome to modify the main method as you please, perhaps
    to add further tests of your own.  We will not be grading it in this
    assignment.  (It does, of course, need to compile.)
3)  You are welcome to add new methods to the Date class.  Since they will
    presumably be "helping" methods, declare them "private", not "public".
4)  Do not not change the prototype (interface) of any method.  If you change
    the arguments or return value, or you change a method from static to
    non-static, your program will not compile with our test cases, and will
    not receive credit.
5)  Do not have any extraneous print statements in your program, including
    error messages.  Your program should print out exactly what is specified
    and nothing else.  (If the comment prefixing a method does not mention
    printing, the method should not print anything.)  The only exception here
    is the main method, which can do anything you like.
6)  Although some test cases are provided in the main method, we will add
    trickier ones to our grading test suite, which won't be run until _after_
    the due date.  It is your responsibility to ensure that your methods work
    correctly on any input, not just the test cases.

The file Date.java contains a skeleton, plus some test code, for a Date class.
Your job is to fill in the implementations of the methods.  We have specified
most or all of the methods you'll need, including some helper methods.

Part I
------
Implement the basic helper methods listed below.  These methods, like the main
method, are declared "static."  They are also declared "public" so we can test
them from within another class.

The Unix "cal" command will remind you of the number of days in each month.
February contains 28 days most years, but 29 days during a leap year.  A leap
year is any year divisible by 4, except that a year divisible by 100 is not a
leap year, except that a year divisible by 400 is a leap year after all.
Hence, 1800 and 1900 are not leap years, but 1600 and 2000 are.  (Implement
this rule in your program even if you know information to the contrary.)

  /** Checks whether the given year is a leap year.
   *  @return true if and only if the input year is a leap year.
   */
  public static boolean isLeapYear(int year) {
    ...
  }

  /** Returns the number of days in a given month.
   *  @param month is a month, numbered in the range 1...12.
   *  @param year is the year in question, with no digits omitted.
   *  @return the number of days in the given month.
   */
  public static int daysInMonth(int month, int year) {
    ...
  }

  /** Checks whether the given date is valid.
   *  @return true if and only if month/day/year constitute a valid date.
   *
   *  Years prior to A.D. 1 are NOT valid.
   */
  public static boolean isValidDate(int month, int day, int year) {
    ...
  }

Part II
-------
Define the internal state that a "Date" object needs to have by declaring some
data fields (all private) within the Date class.  Define the basic constructor
specified below.  A Date should be constructed only if the date is valid.  If
a caller attempts to construct an invalid date, the program should halt after
printing an error message of your choice.  To halt the program, include the
line:

  System.exit(0);


  /** Constructs a date with the given month, day and year.   If the date is
   *  not valid, the entire program will halt with an error message.
   *  @param month is a month, numbered in the range 1...12.
   *  @param day is between 1 and the number of days in the given month.
   *  @param year is the year in question, with no digits omitted.
   */
  public Date(int month, int day, int year) {
    ...
  }

  /** Returns a string representation of this date in the form month/day/year.
   *  The month, day, and year are printed in full as integers; for example,
   *  12/7/1998 or 3/21/407.
   *  @return a String representation of this date.
   */
  public String toString() {
    ...
  }

Part III
--------
Implement the following methods.

  /** Determines whether this Date is before the Date d.
   *  @return true if and only if this Date is before d. 
   */
  public boolean isBefore(Date d) {
    ...
  }

  /** Determines whether this Date is after the Date d.
   *  @return true if and only if this Date is after d. 
   */
  public boolean isAfter(Date d) {
    ...
  }

  /** Returns the number of this Date in the year.
   *  @return a number n in the range 1...366, inclusive, such that this Date
   *  is the nth day of its year.  (366 is only used for December 31 in a leap
   *  year.)
   */
  public int dayInYear() {
    ...
  }

  /** Determines the difference in days between d and this Date.  For example,
   *  if this Date is 12/15/1997 and d is 12/14/1997, the difference is 1.
   *  If this Date occurs before d, the result is negative.
   *  @return the difference in days between d and this date.
   */
  public int difference(Date d) {
    ...
  }

Part IV
-------
Implement the final missing piece of your class, a second constructor that
takes a String argument.

  /** Constructs a Date object corresponding to the given string.
   *  @param s should be a string of the form "month/day/year" where month must
   *  be one or two digits, day must be one or two digits, and year must be
   *  between 1 and 4 digits.  If s does not match these requirements or is not
   *  a valid date, the program halts with an error message.
   */
  public Date (String s) {
    ...
  }

Hint:  use the online Java API to familiarize yourself with all the methods
available to you in the String class.

Submitting your solution
------------------------
Change (cd) to your hw2 directory, which should contain Date.java.  Create a
file called GRADER and put your name, login, lab section, and lab TA's name
there.  If you have any comments you would like to make to the grader, such as
parts of your solution that you know are not correct, include those in GRADER
as well.

From your hw2 directory, type "submit hw2".  After submitting, if you realize
your solution is flawed, you may fix it and submit again.  You may submit as
often as you like.  Only the last version you submit before the deadline will
be graded.