Unidirectional and Bidirectional association

Can someone explain the difference between Unidirection association and Bidirectional association.
An example will be a great help....
Thanx to all of you.
Satya

montefusco,
I believe you are correct in stating that in an information system (implementation) often there is a unidirectional association between two objects where in the Real World (conceptual) there is a bidirectional association.
When I mentioned callbacks I was exclusively thinking of an actual callback impelementation.
In an observer pattern implementation, for example, the Observer object has a reference to the Observable object so that the Observer can register itself to the Observable object. The Observable object has a reference to the Observer object for notifications. (of course there are other ways of implementing this pattern, but you get the point, I hope)
So, I don't think we contradict each other here. There was just a confusion with coneptual-level class diagrams and implementation-level class diagrams.

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Unidirectional vs bidirectional association

Hi,
I've read lots of examples and it seems to me that, in reality, these are bidirectional associations:
Employee and Department (given an employee, you can find out what department(s) s/he belongs to; given a department, you can find out which employees are in it)
Parent and Child (given a parent, you can find out who are his/her children; given a child, you can find out who his/her parent(s) are)
Bid and Item (given a bid, you should be able to tell which item it is for; given an item, you can tell which bids were made on it).
To show unidirectional, the author just omitted the reference to the other in one class. In my opinion, all of the above should be bidirectional (in real life, they would be, I think).
Can someone give me some real life examples of a unidirectional association?
Are most associations bidirectional?
Thanks,
C

Hi,
I've read lots of examples and it seems to me that,
in reality, these are bidirectional associations:
Employee and Department (given an employee, you can
find out what department(s) s/he belongs to; given a
department, you can find out which employees are in
it)
Parent and Child (given a parent, you can find out
who are his/her children; given a child, you can find
out who his/her parent(s) are)
Bid and Item (given a bid, you should be able to tell
which item it is for; given an item, you can tell
which bids were made on it).
To show unidirectional, the author just omitted the
reference to the other in one class. In my opinion,
all of the above should be bidirectional (in real
life, they would be, I think).
Can someone give me some real life examples of a
unidirectional association?
Are most associations bidirectional?You're quite correct - in "real" life, relationships like the ones you cite are bidirectional.
However, that does not mean that every software representation of real life situation require the bidirectional relationship. If the model you've derived has no need of the many-to-one inverse relationship, it's possible to leave it out. Why do it? If the model doesn't require it, looser coupling.
If I'm modeling a parent-child relationship in a financial application, the parent has to know about child dependents, but there might not be any reason for the child object to know who their parent is. I only navigate to children objects by accessing the parent first.
%

Bidirectional association

Hello,
I am designing an application where there are Employee objects and there are WorkBlock objects. A WorkBlock object is simply a block of time that the Employee is scheduled to work.
Each Employee can be scheduled to work one or more WorkBlocks. Each WorkBlock, of course, has one Employee that is assigned to it.
My question is this: I was thinking of having the Employee maintain a List of WorkBlocks that they are assigned to, and also giving a reference to each WorkBlock pointing back to its assigned Employee. This appears to be a bidirectional association, from what I've read in UML book. I thought that this would be a good idea in the long run since you can query the WorkBlock for its Employee and you can query the Employee for its WorkBlocks.
But is this bad code style? Should I instead just have a unidirectional association, and just loop through the entire collection of one item to find the other item?
(For instance, if I only gave WorkBlocks a reference to its Employee, then the only way I could find out which WorkBlocks are assigned to a given employee is to iterate over the entire list of WorkBlocks and look for matching Employees. Does this make sense?)

Erik,
By 'dynamic' I meant that the bidirectional associations can come and go during the lifetimes of the objects involved.
By delegating the job of handling the relationship to something else (the mediator), you can simplify the problem and it's a natural consequence of using the mediator pattern that integrity is maintained. Very nice.
What the mediator does is something like what jschell describes above. How you implement it depends on whether the associations are one-to-one, one-to-many or many-to-many and also it depends on how you want to find or traverse the objects on the other end of the relationship.
Have a look at http://patterndigest.com/patterns/Mediator.html for a quick summary.
One final piece of advice: keep the use of the mediator as an implementation detail shared by the classes that exhibit the bidirectional association. i.e. do not let the rest of the object model become aware of the existence of the mediator. This is often simply a case of getting the object at one end of the association to delegate to the mediator.
Regards,
Lance

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HI and Welcome to the forums!
Here are several KB's that discuss the issue...hopefully one of them has what you need:
KB04254 Network@etp email messages arrive on the BlackBerry smartphone
KB10289 What is a Network@etp message
KB13551 Receiving both email messages and network@etp email messages on the BlackBerry Smartphone
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Good luck and let us know!
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.
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In the interim... For All things TM... See Pondini's Excellent information here...
http://pondini.org/OSX/Home.html
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