Re: Singletons?

"Oliver Wong" <>
Tue, 13 Jun 2006 18:57:03 GMT
"Mark Space" <> wrote in message

I'm trying out some new Java code. I was surprised to find that Java
doesn't allow the static keyword for classes.

    It does for inner classes, e.g.:

class Foo {
  static class Bar {

    but the semantics of the static keyword here have nothing to do (that I
can think of) with the Singleton design pattern.

How was I to implement singleton objects then? Well, a quick web search
and I've got the answer, but now I have a few other questions on the JVM
and compiler.

Let's say I have a class entirely of static methods:

public class anAPI {
static void methodA {}
static void methodB {}

What happens if new is called for anAPI?

    If we're getting into the technical details, I think it's useful to not
think of "new" as something which is called, but rather an operator, the
unary minus is an operator. It takes a constructor as its single argument,
and creates a new instance of the object, then calls the constructor on that
instance. That's my mental model, anyway.

Does any memory get allocated on the heap?

    A new object gets created. Where it gets created is implementor defined,
I think.

And will two such references to anAPI pass the == test?

    If you create two objects via the "new" operator, they will not be
considered the same object, and so the == operator will give false.

It seems to me a smart compiler and JVM would figure out that there no
memory needed on the heap for this object, but maybe things don't work
that way.

    I don't think this particular optimization is allowed to be done, as it
would break the semantics of the == operator.

    - Oliver

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