You are accessing an object in line 80 of Bookstore.java which hasn't been
initialized.

In response to the other answers, technically it's not a matter of an object
being "initialized."  For instance, consider two examples:

MyClass a;
MyClass b = null;
a.doSomething();
b.doSomething();

b has been initialized, though it's just been set to null.  The compiler
will never complain that b might not be initialized.  a has never been
initialized.  So if you write code where a might be accessed at any point,
the code will not compile.  (I think you can use a switch to force
compilation, but that's not wise.)

OK, so what is a NullPointerException?  Ah, the good old days of being a
Java beginner.  I remember when I too did not understand what this meant,
and I asked a reputed Java "expert" what was going on.  He looked kind of
nervous, and pulled down a book on Java and told me to look it up.  Ha!
Some expert.

Anyway, when an object is null, it means it hasn't yet been instantiated.
(Which is a different thing from being initialized.)  Think of it this way.
A variable can be thought of as a mask.  If you say, "Hello there, how are
you?" the mask will just sit their silently.  Now, if you instantiate the
variable, it's like a person wearing the mask.  Now the mask (the person,
actually) can respond to your questions.  So:

StringBuffer a = null;
a.append("a");

Will result in a NullPointerException, because a is just the "mask."  On the
other hand:

StringBuffer a = new StringBuffer; //Put a person behind the mask
a.append("123ABC");

will not result in a NullPointerException.