Tips for developing a06, BreadthFirstApplet.

First, there was a slight error on the original handout, where it said
that the search would be done in the Grid class.  While this can be the
case for a07, where the search itself will be hidden, for the present
problem the search is best done in the BreadthFirstApplet class.
Also AppletGrid.java will need to be modified to accomodate the new
forms of drawing.  It all should be doable in about 200 additional lines total.

On to the tips ...

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For debugging your code, start with initialSampleIndex set to a lower value,
such as 0.  This will get you to the error condition faster and you will
have fewer possibilities to consider.  Of course you may create still 
simpler examples than this 5x5 grid.  Starting with small examples is
a good idea, no matter what you are doing.

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Even though your primary output is graphical, you can still use GOP
(Good Ol' Print statements) for debugging.  For example, if your search
is going astray, you can print out the coordinates of the square you
are examining and find out if they are changing, recurring, etc.

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When using the appletviewer, the output of System.out.println will be
in the usual place, the standard output.  When using a browser it will
be in the "Java Console".  The location of the console differs from
browser to browser and version to version.  Some vendors are not happy
unless they put it in a different place in each version, so you have
to be willing to look around.  Here are some known locations:

    Internet Explorer 5.2: Pull down the View Menu, and select Java Messages.
    (The last time I checked on the Mac version, this message window was 
    cumulative and I could find no way of clearing it, so be prepared 
    to view all the messages created by your applets since the beginning of 
    time.)

    Netscape 7.0: Go to the Tools menu, select Web Development, then
    Java Console.

    Apple Safari: Let me know if you find it.

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The Grid/Square classes are set up so that "marking" a Square is equated
to "having a parent".  If you use this, then the start Square of the search
will need a parent.  But it really doesn't have one, so make a bogus one,
as Pair(-1, -1) for which you can test later.  Similarly, you might want
to make the spam Square found have a parent for purposes of reconstructing
the path.

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Enqueue coordinate pairs of Squares, not direct references to Squares.

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The logic for deciding whether one of the four neighbors of a square should
be enqueued is slightly complex, involving a number of tests.  Rather than
replicate this logic 4 times, make a method that carries it out for a
generic pair of coordinates.  Only if the tests are passed is the coresponding
Square enqueued.  So I call this method "maybeEnqueue".  Then I call this
method 4 times, one for each of the offset coordinate pairs.

I have my method check for spam and return a boolean so indicating.  But
you can instead check when dequeuing.  If you put it in the method, you
probably should check for the start square containing spam as well.
I do this by enqueuing the start square in the same method.  But when
we do the DESC in a07, the start square will never have spam, so it is not
a big thing.

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What do you do when you've found a path, or determined that none exists?
Don't break out of step() or the run() method (in BaseApplet) that calls it.
Rather, just set a boolean that tells you the path has been found.  Test
this method inside of step and devise your display to display one way or
another depending on this flag.  Because I display my path incrementally,
I have a second flag indicating whether the path has been completely
displayed.  And I have a third display mode for when there is no path.
If you want to just display the whole path at once non-incrementally, that
is ok too.

So your applet never really stops running (until you leave the page).
If a new sample is selected, the search is reinitialized and starts all
over.  All the time the display is being driven by the run() method
calling the start method.  (If you really want to get out for some
reason, you can over-ride the continue() method in BaseApplet.)