Re: Life of an instance

Eric Sosman <esosman@ieee-dot-org.invalid>
Sat, 02 May 2009 13:44:52 -0400
Salad wrote:

Mark Space wrote:

Salad wrote:

I might have
    Customer cust = new Customer()

So how long does cust live? Is there a way to close or kill off
cust? Or does the instance live for as long as the application runs?

"cust" lives until it is no longer reachable.

  void test1 () {
    Customer cust = new Customer();
    System.out.println( cust );

"cust" is no longer reachable at the closed brace "}" and is available
for garbage collection there.

There's no real need to "close" objects premature. The Java language
defines when they go out of scope, it's done (for the most part)
automatically for you.

If you have a specific situation, then please describe it in detail.
There are a few corner cases where the programmer one must assist the
JVM, or one can hold a reference too long and prevent the JVM from
garbage collecting object. But those are more "debugging" scenarios,
not really a general principle.

I didn't have a specific situation. I was thinking that if one had,
let's say 3 customers to process, if one reused the object cust (like in
a for loop) or if 1 held all three custs, a different instance for each

     Here's a thought that may clarify things: `cust' is *not*
a Customer instance, but a reference that points to a Customer
instance. At different times in your program, the variable
`cust' may point to different Customers: "John Doe," "Joe Schmo,"
and "Dead Beat," for example. It may also have the value `null',
pointing to no Customer at all.

     A given Customer instance remains "live" as long as it can
be reached by some chain of references, somewhere. For example,
as long as `cust' points to "John Doe," the "John Doe" Customer
remains live and will not be garbage collected. But if you then
change `cust' to point to "Dead Beat" instead, it no longer refers
to "John Doe." If there are no other references to "John Doe,"
the garbage collector is free to reclaim it.

     The `cust' variable itself also has a lifetime, independent
of the lifetime(s) of the Customer(s) it might point at from time
to time. In the test1() method above, `cust' comes into existence
when the sequence of execution reaches its declaration, and goes
out of existence at the `}' that terminates its scope. In

    for ( ... ) {
        Customer cust = ...;

the `cust' variable comes into existence each time its declaration
is reached, and goes out of existence each time the `}' is reached
(treating the `continue' statement as if it goes to the `}' before
going back to the `for' again). That is, there is a new `cust'
for every loop iteration.

     What happens to the Customer that `cust' (any of them) refers
to is another question. If the `cust' was the only reference to
the Customer, the Customer becomes garbage as soon as the `cust'
goes out of existence or otherwise ceases to refer to it. If you
have saved another reference to the Customer somewhere -- in a
Collection, for example -- then its lifetime does not depend on
whether `cust' still points to it: As long as *any* live reference
still points to the Customer, the Customer remains alive.

Eric Sosman

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