Suggestions and Hints for Project #1

Last modified: 9/20/2002.

0. Be sure to look at the posted project updates.

1. Take a look at the definition of the standard Java StringBuffer class.
   You may find it useful for implementing toString methods.

2. You may find ArrayList (or Vector) and  LinkedList in java.util useful.
   
3. When copying arrays, ArrayLists, LinkedLists, etc., whose items have
   reference types, always remember that you are ONLY COPYING POINTERS!
   If you copy an array of Polynomials, for example, and then change a 
   field in one of the Polynomials in the original array, that field
   also changes in the corresponding item of the copy.  So, if you
   DO introduce methods that change a Polynomial's fields (or a Term or
   Factor, if you introduce such types), always be aware of what other
   Polynomials (that you intended to be separate) might get affected 
   by any change you make.   

4. Handling expressions like 'x-3y' is a little tricky.  A StreamTokenizer
   will return 'x', the number -3, and 'y'.  According to the handout, 
   you are supposed to treat this as x+(-3y), not as -3xy.  It's not
   too difficult---just remember that a negative number marks the end 
   of one term and the start of another, whereas a non-negative number
   does not.

5. The field inp.ttype is of type int.  However, when you read an
   "ordinary" character (punctuation, lower-case letters), it is supposed
   to represent a character (its value will be the numeric ASCII code used 
   for that character: for example 'a' will show up as 97).  Just remember
   that you may need to tell Java (or GJDB) to treat inp.ttype as a 
   character for some purposes, using a cast.  For example, in GJDB, 
   you might write	

	print (char) inp.ttype

   Be especially careful in printing or in adding to a string.  For example,
   if I've just read an 'x', then the String
	"I just read " + inp.ttype
   is "I just read 97".  You probably meant
	"I just read " + (char) inp.ttype, 
   which prints "I just read x". 

6. The type char is one of the numeric types (see PIJ 6.3).  All characters
   in Java are represented by numbers according to a standard known as
   Unicode.  Conveniently, the letters 'a'-'z' are represented by 
   consecutive numbers, as are 'A'-'Z' and '0'-'9'.  That means that 
   expressions such as x - 'a' will convert a letter a-z stored in 
   int or char variable x into a number in the range 0-25 .  The reverse
   translation (which you won't need in this projectx) is (char) (n + 'a').

7. In GJDB, when you do a 'next' instruction over a read (such as
   inp.nextToken ()), then nothing will seem to happen (the computer won't 
   respond).  That's because it's waiting for input from you, since it is
   executing an input statement.  HOWEVER, the Java runtime generally reads 
   an entire line at a time.  So if you have an input loop:

	while (...) {
	   inp.nextToken ();
	   ...
   
   then GJDB will wait for input the first time you try to step over the
   'nextToken' call, but if you then type a line with several tokens, like

	x - 3

   it will NOT wait on the second and third iteration, because the program
   will still have leftover unprocessed input to work through.

8. Try to get your toString methods done as soon as possible.  They are
   convenient for use in GJDB. For example, we stopped at the beginning
   of 

     /** Returns THIS + Y.  THIS is not affected. */
     public Polynomial add (Polynomial y) {
        ...

   you can look at this and at y with

	main[0] p y.toString ()
        main[0] p this.toString ()

   or even 

	main[0] p this + "+" + y

   Of course, these are less useful if toString doesn't work properly.