Re: Java collections

tim@nocomment.com wrote On 05/03/06 15:44,:


    It seems you're still a little bit confused, so let
me try to hammer home a point that I think will clear
things up for you. Chisel the following on a suitable
slab of marble and hang it on the wall at your workplace:

               VARIABLES ARE NOT OBJECTS

    Now, what on Earth could I possibly mean by this
nonsense? Of course, variables are (or can be) objects!
When I write `int i' it means "the variable i is an int,"
so when I write `Customer c' it must mean "the variable c
is a Customer." I must be wacko to think otherwise, right?

    Well, no: Analogy is not proof, and two syntactic forms
that look similar don't necessarily mean the same thing.
`Customer c' means that c is a *reference* to a Customer,
not that c *is* a Customer. The Customer instance itself
lives somewhere out in hyperspace, and c holds a reference
to it. When you do

    Customer c1 = new Customer("Tim");
    Customer c2 = c1;

.... there is only one Customer instance in play, and both
reference variables c1 and c2 point to it. If the above
is followed by

    c2 = null;

this does not destroy the Customer instance, nor influence
it in any way: the instance lives on, even if one of the
many references to it has disappeared. (Write your phone
number on a piece of paper, then burn the paper: Did your
phone catch fire?)

    Now: When you call collection.add(c1), what manner of
argument does the add() method receive? You're catching
on: it receives a copy of the reference c1, so the argument
is yet another reference to that lonely Customer instance.
What do you suppose add() puts into the collection? Yes,
it salts away a copy of its argument, that is, a reference
to that very same Customer. After add() returns, there are
two references to the Customer in your program: the variable
c1 and another one somewhere in the colleciton. (c2, you'll
recall, has been set to null and no longer refers to the
Customer.)

    So:

    - Making lots of copies of a reference doesn't make
lots of copies of the Customer it refers to. (Photocopying
your phone number doesn't clone your phone.)

    - If you make a change to the Customer, that change
becomes apparent via all the references. (Change the "I'm
busy now" message on your phone, and all callers will hear
the new message even if they got your number from an old
piece of paper.)

    - Any reference to a particular Customer is just as good
as any other; they all work identically. (It doesn't matter
which piece of paper somebody reads to find your phone number;
your phone rings anyhow.)

    The last point also applies to the garbage collector: as
long as at least one reference to the Customer survives, Java
knows that the Customer might still be used and therefore will
not treat it as garbage. Since all references are equivalent,
a reference inside a collection is every bit as effective as
an ordinary reference variable: as long as the collection still
refers to the Customer (and as long as the collection itself
remains accessible), the Customer is accessible and Java won't
discard it.

    Concentrate on the distinction between references and
instances, and I think much of your confusion will disappear.

--
Eric.Sosman@sun.com