Don't worry too much about it - these are only warnings, and occur because  
some of the code does some slightly hairy stuff with typing. Reasons  
include:

    + It was written when no alternative means was available.

    + It deliberately avoids type safety for the sake of example.

    + The programmer made a (often valid) decision that he knew something the  
compiler didn't.

If you add the -Xlint:unchecked argument to the javac command, you'll see  
a detailed list of the warnings. It's probably best to ignore them for  
now, though, and concentrate on the basic concepts from your book.

That's because Sun introduced some extensions into the most recent version of
Java, and the compiler is trying to force you to use them. Leaving aside the
question of whether the extensions are a good thing or not, they are a
complication that you don't need at this stage, and especially not while you a
trying to follow a book which pre-dates them.

The easiest way of sorting the problem is just to ignore the messages ;-) but
if you prefer you can tell the compiler that you are writing in "old" Java:

   javac - source 1.4  -classpath ....etc...

   -- chris

Chris's advice is practical. If you want to see the errors type

javac -source 1.5 -target 1.5 -Xlint:unchecked *.java

That will show you the warnings, which will have to do with missing
generic declarations.  If you feel up to tackling them, read
http://mindprod.com/generics.html
Otherwise, for now, just ignore the warning messages about "unchecked"
stuff.