OO Design: Who can do what?

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antonyliu2002@yahoo.com - 10 Aug 2007 16:21 GMT

OO Design continued.

When we try to design a class, we try to think about what the objects
of the class can do, like quite many OO programming books say.

Let's suppose that we are gonna design a ProjectManager class and a
Project class.

Well, what can a ProjectManager do? I think s/he can
createNewProject, deleteProject, assignFundToProject, requestFund,
deleteFund, editProject, viewProjectList, etc.

What can a Project do? I don't know except some getter setter
methods. The reason I don't know about it is because a project is not
animate, thus I am not sure what action it can perform.

So, naturally, I put those named methods above (createNewProject,
deleteProject, assignFundToProject, etc.) into the ProjectManager
class, because I think those are the things that a ProjectManager can
do.

But my colleague told me that those methods should really go into the
Project class. I assume that my colleague is right, because I do not
have much experience in OO design. But, then,

1. Why does it make more sense to put those methods in the Project
class than in the ProjectManager class, giving the instructions of OO
programming textbooks?

2. If we move those methods into the Project class, then what can a
ProjectManager do?

I want to be a good OO designer, so, please do share your wisdom about
this. Thanks.

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Manivannan Palanichamy - 10 Aug 2007 17:46 GMT

On Aug 10, 8:21 pm, "antonyliu2...@yahoo.com"
<antonyliu2...@yahoo.com> wrote:

> OO Design continued.
>
[quoted text clipped - 30 lines]
> I want to be a good OO designer, so, please do share your wisdom about
> this. Thanks.

Your first design is correct. the design confusion is because of
unclear requirements. you should start from UseCase diagram, and then
will come to know what each Actor got to do.

Anyway. In this case, the Project will be a a resource class. Like a
value object. So, obviously it will have only getters & setters.

I would agree with the following design:

class Project
{
get/setName();
get/setReleaseDate();
get/setTeams();
get/setProjectManager();
}

class ProjectManager
{
Project createNewProject();
deleteProject(Project project);
assignFundToProject(Project project, Fund fund);
}

--
Manivannan.Palanichamy (@) Oracle.com
http://mani.gw.googlepages.com/index.html

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antonyliu2002@yahoo.com - 10 Aug 2007 18:03 GMT

On Aug 10, 12:46 pm, Manivannan Palanichamy
<manivannan.palanich...@gmail.com> wrote:

> On Aug 10, 8:21 pm, "antonyliu2...@yahoo.com"
>
[quoted text clipped - 62 lines]
> --
> Manivannan.Palanichamy (@) Oracle.comhttp://mani.gw.googlepages.com/index.html

Interesting, so you are with me, and disagree with my colleague. Any
comments from other OO gurus?

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Alexey - 10 Aug 2007 18:36 GMT

On Aug 10, 1:03 pm, "antonyliu2...@yahoo.com"
<antonyliu2...@yahoo.com> wrote:

> On Aug 10, 12:46 pm, Manivannan Palanichamy
>
[quoted text clipped - 68 lines]
> Interesting, so you are with me, and disagree with my colleague. Any
> comments from other OO gurus?

I tend to agree with your and GP's design. Generally speaking, it IS
a good idea to assign "verbs" as methods to the actors. Other methods
that Project may have would come no so much from actions, but perhaps
requirements that detail what kind of information might be needed from
within the Project entity. For example:

class Project
{
// ... getters and setters for simple data fields

Date getCompletionETA()
Money estimateCost()

// some task management
Set<Task> getTasks()
Set<Task> getTasksAssignedTo(Employee emp)
}

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antonyliu2002@yahoo.com - 10 Aug 2007 19:24 GMT

> On Aug 10, 1:03 pm, "antonyliu2...@yahoo.com"
>
[quoted text clipped - 90 lines]
>
> }

Cool, sounds like that I have been correct, but my colleague was
wrong. Feeling so proud. So, the principle is really simple: assign
"verbs" as methods to the actors.

Reply to this Message

Mike Schilling - 10 Aug 2007 21:03 GMT

> Your first design is correct. the design confusion is because of
> unclear requirements. you should start from UseCase diagram, and then
> will come to know what each Actor got to do.
>
> Anyway. In this case, the Project will be a a resource class. Like a
> value object. So, obviously it will have only getters & setters.

Quibble:

Suppose one of the things a project manager can do is combine two project,
merging all of the resources associated with project A into project B.
Agreed that there should be

ProjectManager.combineProjects(Project mergeInto, Project mergeFrom)

I don't think that this should be implemented by a list of get() and set()
calls. The knowledge of what resources exist and how to merge them (as well
as what state the cannibalized project goes into) belongs in the Project
class. That is, if I add a new sort of resource to Project, it should be
clear in the Project class what corresponding changes need to be made.

Thus this should be implemented as

combineProjects(Project absorbInto, Project cannibalizeFrom)
{
absorbInto.absorb(cannibalizeFrom);
}

with the details of this processing in Project.absorb(). (Obviously, if
the two projects were combined into a brand-new project, the work would be
done in a Project constructor.)

Reply to this Message

Greg R. Broderick - 11 Aug 2007 18:17 GMT

> Well, what can a ProjectManager do? I think s/he can
> createNewProject, deleteProject, assignFundToProject, requestFund,
> deleteFund, editProject, viewProjectList, etc.

IMO, you've neglected one very important, frequently-invoked method for the
ProjectManager class. I've provided a reference implementation below.

public class ProjectManager
{
...

public void pesterWorkers()
{
System.out.println("What do you mean it isn't done yet?");
}
}

Cheers
GRB

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antonyliu2002@yahoo.com - 14 Aug 2007 17:01 GMT

On Aug 11, 1:17 pm, "Greg R. Broderick"
<usenet200...@blackholio.dyndns.org> wrote:

> "antonyliu2...@yahoo.com" <antonyliu2...@yahoo.com> wrote innews:1186759275.447652.206350@d30g2000prg.googlegroups.com:
>
[quoted text clipped - 26 lines]
> Q. What is the most annoying thing on Usenet?
> ---------------------------------------------------------------------

Ha, that's a good joke. My Project Manager has been nice so far.

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TobiMc3@gmail.com - 14 Aug 2007 19:21 GMT

On Aug 11, 1:17 pm, "Greg R. Broderick"
<usenet200...@blackholio.dyndns.org> wrote:

LOL-Great minds!!

Tobi

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TobiMc3@gmail.com - 14 Aug 2007 19:17 GMT

On Aug 10, 11:21 am, "antonyliu2...@yahoo.com"
<antonyliu2...@yahoo.com> wrote:

> OO Design continued.
>
[quoted text clipped - 16 lines]
> class, because I think those are the things that a ProjectManager can
> do.

An aside:

I think you forgot yellAtDeveloper() :-)

Tobi

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