Mailing-List: contact users-help@groovy.apache.org; run by ezmlm
Precedence: bulk
Reply-To: users@groovy.apache.org
Content-Type: text/plain; charset=iso-8859-1
Mime-Version: 1.0 (Mac OS X Mail 6.6 \(1510\))
Subject: Re: Hierarchy of singletons howto?
From: OC <ocs@ocs.cz>
In-Reply-To: 
 <CAL2Kp+jRevF95YgwJ7uZfugFPdbMZs_P_-Y5e1KJNaYfDDGyeg@mail.gmail.com>
Date: Fri, 1 Apr 2016 21:38:43 +0200
Content-Transfer-Encoding: quoted-printable
Message-Id: <1A523D17-D575-461A-9274-3E3D831607DD@ocs.cz>
References: <F52DEED4-9584-4476-B621-69AD01F0E77B@ocs.cz>
 <CAL2Kp+jRevF95YgwJ7uZfugFPdbMZs_P_-Y5e1KJNaYfDDGyeg@mail.gmail.com>
To: users@groovy.apache.org

At least one of us two must have missed something in =
Object-Oriented-101.

There is exactly one instance of class Foo.

And there is exactly one instance of class Bar, whose functionality is =
derived from Foo.

Consider the factory pattern: there are two related classes, A and B =
extends A. If it so happens that each of them needs a factory class, =
then is is self-evident that

(a) both AFactory and BFactory need to be singletons (as any factory out =
there)
(b) BFactory should extend AFActory, for just as B's functionality =
extends A's functionality, the same applies for their factories.

And one can implement the functionality easily, e.g., using

=3D=3D=3D
class Foo {
  static instance=3DnewInstance()
}
class Bar extends Foo {
  static instance=3DnewInstance()
}
=3D=3D=3D

it works like a charm, only -- unlike @Singleton -- it is not lazy (and =
if turned to lazy, it would not be threadsafe).

Thanks and all the best,
OC

On 1. 4. 2016, at 11:48, Alessio Stalla <alessiostalla@gmail.com> wrote:

> Your requirement is logically inconsistent. If there is only one =
possible instance of Foo, there cannot be /another/ instance of Foo =
which is also a Bar.
>=20
> On 1 April 2016 at 04:28, OC <ocs@ocs.cz> wrote:
> Hello there,
>=20
> how do you make a hierarchy of classes, each of which happens to be a =
singleton?
>=20
> The na=EFve solution simply does not work:
>=20
> =3D=3D=3D
> 85 /tmp> <qq.groovy
> @Singleton class Foo { }
> @Singleton class Bar extends Foo { }
>=20
> println "Foo: ${Foo.instance}, Bar: ${Bar.instance}"
> 86 /tmp> groovy qq
> Caught: java.lang.IllegalAccessError: tried to access method =
Foo.<init>()V from class Bar
> java.lang.IllegalAccessError: tried to access method Foo.<init>()V =
from class Bar
>         at Bar.<init>(qq.groovy)
>         at Bar.<clinit>(qq.groovy)
>         at qq.run(qq.groovy:4)
> 87 /tmp>
> =3D=3D=3D
>=20
> What is the proper way to achieve this?
>=20
> Thanks a lot,
> OC
>=20
>=20