Inheritence and kind of doing it backwords.

[Mitch]

Hi.

My problem is quite specific, but doing a search on inheritence was
just too much to sift through.

This post IS very long, and I am sorry, but I've gone for clarity
rather than brevity

I'll try to explain my specific problem in detail, though I expect the
answer is quite simple and common, for that I apologise.



I am writing a rail simulation piece, with each segment of rail an
object. I am then extending the railSection class to have more
specialised railSection objects, such as Signal and Point (to Americans
i think they are called intersections or junctions)...

Now if i have a 'train' running along the track (which is currently an
array or map (prob array) of railSections, how will i test to see if
say a signal is red?

I have a line which currently reads:

///////

while(this.currentRailSection.whatIsSignal() == signalState.RED)
this.currentRailSection.redSignal.await();

////////

Now obviously the 'currentRailSection' could be an extended Signal
railSection or just a standard railSection, and as the compiler see's
it as a standard railSection it flags a message.

Now if the Signal class inherited the 'signalState' from the
railSection i imagine all would be fine, however that is not how i
would like it to be. So i guess my question is how do I test for
something in an extended class when i have to refer to it as the base
class...?


I think an abstraction:

////////
class a
{
blah
}

class b extends a
{
blah...

private variable whatIsMyVariable() { return _variable; }

private variable _variable;
}

-=-=-=-=-=-=-

Now i want to do

while(a<100; a++)
IF(a.whatIsMyVariable == 123)
do something;

Knowing that it will return false whenever the class a ISN'T actually
b..

////////


So i would have an array which would be my route which would go
something like

railSection
railSection
railSection
railSection
Signal extends railSection
railSection
railSection
railSection

And prior to carrying onto the next railSection i want to test to see
if my signal is red.


Hope I have made myself clear and thank you to all who take the time to
read it, I know it's a beast.

Kind Regards,

Mitch.

 

[Mitch]

I'd like to point out that with the line:

///////

while(this.currentRailSection.whatIsSignal() == signalState.RED)
this.currentRailSection.redSignal.await();

////////

It isn't what is in the while statement that is the problem, that was
easily overcome by adding a whatIsSignal method in the railSection
class which returned signalState.NONE

It the is the 'condition' redSignal that is flagging errors.

this.currentRailSection.redSignal.await();

Although this line only executes if the while() is valid it still
errors in the compiler and as such I need to fix it.

 

[Andrew McDonagh]

Hi.

My problem is quite specific, but doing a search on inheritence was
just too much to sift through.

This post IS very long, and I am sorry, but I've gone for clarity
rather than brevity

I'll try to explain my specific problem in detail, though I expect the
answer is quite simple and common, for that I apologise.



I am writing a rail simulation piece, with each segment of rail an
object. I am then extending the railSection class to have more
specialised railSection objects, such as Signal and Point (to Americans
i think they are called intersections or junctions)...

snipped..


one quick observation...... The Train (driver) needs to know what the
signal is... not the tracks.

The signals are not special track segments...

andrew

 

[Paul Hamaker]

You could do something like this :
if ( baseref instanceof Signal ) {
sigref = (Signal) baseref;
if ( sigref.getSigState() ) etc...

 

[Mitch]

I agree in some respects. For instance the code you see above is found
inside my Train class. I.E. it is the train that is checking for the
red signal and then waiting accordingly.

I don't disagree that the signals are not special track segments
either, not in reality, but there are two major points to be considered
from my simulation's point of view. The first is that I need to
specify routes for the trains. Therefore i need for the train to have
an idea of which part of the track it is on and which part of track is
next. So in the example above where i say i need to specify the route
as
railSection
railSection
railSection
signal
railSection
.....

It would be considerably easier for me, my marker, and for modularity
if signal was infact extended from the railSection class. It makes
route specification, train location etc a lot easier.

My second point is that the railSections for the most part also control
the signals. IE if there is a train on the next section of track you
want the preceeding signal to be red (This is specified, and is how it
is done R/L in Britain and most if not all over the world).

For the most part however it boils down to route planning. I need to
specify an array of railSections for the train to travel along, and
adding sections that aren't rail will cause me troubles, hopefully less
than the troubles this condition is causing me anyway.

Thanks for your observations though

 

[Mitch]

"instanceof" is an operator I haven't heard of before and could well be
what I'm looking for, thanks!

 

[Mitch]

Perfect!

I have turned what i had:

////////

while(this.currentRailSection.whatIsSignal() == signalState.RED)
this.currentRailSection.redSignal.await();

////////

into this

////////

if(this.whatIsCurrentRailSection() instanceof Signal){

signalReference = (Signal)this.whatIsCurrentRailSection();

while( signalReference.whatIsSignal() == signalState.RED){
signalReference.redSignal.await();

}

}

////////

And all seems to work perfectly!

Thank you very much, was exactly what I was looking for!

 

[Chris Uppal]

"instanceof" is an operator I haven't heard of before and could well be
what I'm looking for, thanks!

It isn't. If you control all aspects of a design, and find yourself using
instanceof (except in an equals(Object o) method), then it's almost certainly a
symptom of poor design.

-- chris

 

[Chris Uppal]

My second point is that the railSections for the most part also control
the signals. IE if there is a train on the next section of track you
want the preceeding signal to be red (This is specified, and is how it
is done R/L in Britain and most if not all over the world).

How about this:

Say you have a base class called TrackElement. You have subclasses called
RailSegment (representing a stretch of actual rail), Signal, end whatever.

You might have a collection of all the TrackElements, but the Train does not
access that collection directly. Instead each RailSegment knows which
Signal(s) it controls and which RailSegment(s) it connects to. The Train knows
which RailSegment it is on, and asks that for the Signal controlling it's
movement in whichever direction it wants to go. If that Signal is green, then
the Train asks the RailSegment for the next RailSegment and moves on to that.

There are other ways of setting it up -- for instance the Train could ask the
Signal for RailSegment to which it controls access, rather than asking the
RailSegment it is currently on. I think that's rather neater, on the whole.

Thus the Train never traverses any Signals, only RailSegments; and never
checks the status of a RailSegment, only of Signals.

-- chris

 

[Mitch]

Ok I have yet to see a problem but I have also yet to test it.
The code supplied below seems to overcome my problem but it may cause
me another.

////////

if(this.whatIsCurrentRailSection() instanceof Signal){

signalReference = (Signal)this.whatIsCurrentRailSection();

while( signalReference.whatIsSignal() == signalState.RED){
signalReference.redSignal.await();

}

}

////////

This boils down to how Java works internally.

At the bottom of that class I have private Signal
signalReference; and then in the code above I say

when reference = RED
await();

Await is a method relating to conditions in threads. If the reference
is RED then I await another thread to .signalAll() (which happens when
another thread changes a signal from red to something else) and this
then forces the when reference = RED check to check again to see if in
fact it was our red signal that was changed.

Now my question is Once i have created signalReference, will the
condition always be the same. (I.E. Have i copied the values of the
Signal object which means they will never change) or will they change
as well?

 

[Mitch]

I definately agree that your way is better design, however I simply
can't do it that way. This is a programming project, but fortunately
or unfortunately I'm not yet sure, it isn't going to be marked by any
programmers, and only be seen in a report and a 15 minute presentation.

It boils down to lack of time. I have two weeks in which to finish
something I can display to my lecturers. Scalability is a term they
will know and I can play on. If I have a route which is initiated as
an array (maybe read in from a text file) then it will be easy to show
them the route, and easy for me to then tel them how easy it would be
for them to change the route (say for rail maintenence). Doing it your
way, although better, would be more difficult to explain, and I wouldn'
be able to show them how to add / modify a route in the time of the
presentation.

I will at least mention your way if you permit in my report as a better
way of doing things? ( I will have to reference this topic anyay)

Cheers!

Mitch

 

[Chris Uppal]

It boils down to lack of time. I have two weeks in which to finish
something I can display to my lecturers.
Unfortunate...


I will at least mention your way if you permit in my report as a better
way of doing things? ( I will have to reference this topic anyay)

Sure. Be my guest.

(Really, there was no need to ask -- everything on Usenet is public anyway --
but I appreciate your courtesy.)

-- chris

 

[Patricia Shanahan]

Mitch wrote:
....

Now obviously the 'currentRailSection' could be an extended Signal
railSection or just a standard railSection, and as the compiler see's
it as a standard railSection it flags a message.

....

A signal is not a type of rail section. A signal is something some, but
not all, rail sections have.

I would move out all code related to signal management to a separate
Signal class, not derived from RailSection. It is possible signalState
should disappear into the reformed Signal:

while(currentRailSection.hasSignal() &&
currentRailSection.getSignal().whatIsSignal() == Signal.RED)


If you make Signal a subclass, how would you handle switches that have
signals? It's trivial if you treat having a signal as an optional
attribute of any rail segment.


Patricia

 

[Roedy Green]

"instanceof" is an operator I haven't heard of before and could well be
what I'm looking for, thanks!

see http://mindprod.com/jgloss/instanceof.html

 

[Andrew McDonagh]

Mitch wrote:




It isn't. If you control all aspects of a design, and find yourself using
instanceof (except in an equals(Object o) method), then it's almost certainly a
symptom of poor design.

-- chris

very strongly seconded!

 

[Chris Uppal]

when reference = RED
await();

Unfortunately that isn't Java so I don't know exactly what the pseudo-code is
supposed to mean.

Now my question is Once i have created signalReference, will the
condition always be the same. (I.E. Have i copied the values of the
Signal object which means they will never change) or will they change
as well?

If the signalReference variable holds a reference to an object (its an odd name
for a variable, btw), and if you don't assign a ref to a different object to
it, then it will (this is kinda obvious ;-) still refer to the same object. If
the state of that object changes over time, then whenever you use that variable
it will still contain a reference to the same object, and so you will see that
object's current state each time.

E.g:

Signal signal = someSignalOrOther();
signal.isRed(); // returns true

... time passes during which other code executes on
... another thread and changes the state of the signal

signal.isRed(); // returns false

Does that answer your question, or have I misunderstood you ?

-- chris

 

[Mitch]

Chris, you understood with that last post, and yeah it did answer so
thank you.

Also thank you Roedy, I had found that after the recommendation.

And Patricia thank you for your comments.
As I said to Chris before there is the unfortunate problem of this
being marked by non-programmers and so I am trying to use as many key
topics as I can, one of which is inheritence. That is why I have done
it that way. My original code had one RailSection class with
attributes such as hasSignal and hasPoint. I changed it with good
reason, though it seems to have lost me somewhere. I am however
strongly considering re-writing the class at the moment. I am having
troubles with Conditions which are new to me and I think simplifying to
overal structure could help.

Thank you all for your help and I will report back here as to what I
do,

Kind Regards,

Mitch.

 

[Mitch]

Patricia.

Having started viewing these groups a lot lately i decided to go and
get myself a newsreader and stop using google groups. Your post must
have been in this transition because I have only just noticed it. (I
am searching for those relating to my project to reference, in case you
were wondering.)

That is indeed a better way of doing it, I only wish I had seen it
sooner. The instanceof method is working for me reliably, however, and
I haven't the time to change it now but thank you anyway.

Kind Regards,

Mitch.

Regarding the switches (and stations) that have signals, they are
placed immeadiately before / after / where they should be in the system
and know of local rail sections so I have found a way around it.

eg

RailSection
RailSection
Signal
Point(switch)
/ \
RailSection RailSection

Thanks again!

Mitch.