Re: Immutable cyclic graph with dependency injection

From:

Owen Jacobson <angrybaldguy@gmail.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 29 Jun 2009 15:53:19 -0400

Message-ID:

<2009062915531975249-angrybaldguy@gmailcom>

On 2009-06-29 15:48:53 -0400, Tom Anderson <twic@urchin.earth.li> said:

On Mon, 29 Jun 2009, Owen Jacobson wrote:

On 2009-06-29 06:47:46 -0400, Philipp <djbulu@gmail.com> said:

I have a structure with two classes, let's call them Car and
SteeringWheel for the example. Car /owns/ a SteeringWheel: in
production code, the lifetime of the SteeringWheel is directly
dependent on the lifetime of the Car. In my design, the SteeringWheel
object needs a reference to the Car object (let's say, to transmit when
the user honks).

I see several ways to build and initialize this class-graph, but none
so far that I think perfect.

If you take a strict view of immutability, you can't initialize an
immutable, circular structure. You need to introduce some formal
mutability somewhere, even if the result behaves as if it were
immutable.

Whilst i wouldn't call this strict, or a good idea (seriously - not *at
all*), you *can* change final references after construction - you can
do it with sun.misc.Unsafe. But of course, that's not a feature of java
:).

Once upon a time, the JVM itself treated final identifiers like any
other identifier in the same category, and allowed bytecode that
directly modified final locals, fields, etc.. javac makes assumptions
about final fields (by inlining constant expressions) that rely on that
never happening.

I have no idea, nor do I want to know, if that's still true.

-o