cs230 Lecture Notes
Week 8 Tuesday


Circular buffer
---------------

For now we are implementors.  In two
assignments we are going to use the queue implementation.

Ideally, we should write and debug it now, and then in
two weeks we can just use it as if it were built in.

Sorry about the confusion in the book: everywhere the
variable last appears, replace it with next.

The instance vars of the Queue are

    first: the head of the queue
    next: the next available space in the queue

Special case, if first == next, the queue is empty.

What happens when first reaches the end of the array?

     Does that mean the queue is full?

     Not necessarily -- we can wrap around to the beginning.


Modulus magic
-------------

The modulus operator performs clock arithmetic.

(Also useful for checking divisibility and extracting the
rightmost digit of a number.)


Circular buffer resumed
-----------------------

So how do we know when the buffer is full?

What do we do when that happens?


Process objects one more time
-----------------------------

Look at Homework 5 solutions

Balance   =   client code  =     Test or Calculator
Checker   =   process object  =  Evaluator


Goals:

1) hide implementation details from client

2) all methods in Evaluator should be able to access the state


Solution:

1) use instance variables to store state


Accidental benefit: we can have multiple processes going on
		    at the same time.

Client view of a process object:

1) create it by invoking new

2) drive the process by invoking methods

Examples:

StringTokenizer: initialize it with a string, invoke hasMoreTokens
		 and nextToken

Checker: initialize with nothing, invoke checkLine and endOfFile

Evaluator: initialize with the expression, invoke evaluate

	   (all other methods can be private, since the client
	   doesn't have to invoke them)