I could have a class that has a port connection and is able to provide to a single listener. However the thing listening also wants to be able to send things out on the port so needs a reference to the port (whether directly or via an interface)

interface Listener
{
void heardSomething(String thing);
}

class Port
{
Port(Listener listener);
void write(String stuff);
}

class ClassA implements Listener
{
ClassA(Port writeTo);
}

Now when it comes to creating this, I need to be able to pass ClassA to Port (as an interface) and Port to ClassA as follows:

main
{
ClassA a = new ClassA(Port…?)
Port p = new Port(a);
}

Now I know that this could be loosened up by allowing more than one listener and rather than passing in the constructor using an AddListener approach, but I want to see how you would work this with the 3rd class approach.

Matt

The relation is not like this. Actually you have this:
Teacher{
List teachedCourses
}

Course {
Teacher teacher
List attendingStudents
}

Student {
List myCourses
}

So t -> s and s -> t are derived relations. You wouldn’t pass S to T nor T to S in the constructor.
In the next step I would dispute bidirectional references above…

Oleg

How would you work around this?

I’ve been playing with Apache wicket and ran into this problem. Here’s the scenario:

PageBorder – a border component that wraps around a BasePage and does stuff like setting title, meta etc.

class PageBorder extends Border {
public PageBorder (id, BasePage page){
this.page = page;
add (“title”, new Label(“title”, page.getTitle());
}
}

At the same time, since the BasePage contains the PageBorder, the BasePage needs to know about the border (create the border passing this as a ctor param)
public abstract class BasePage extends Page {
PageBorder border;
public add (Component c) {
if (border == null) {
border = getBorder();
// subclass must create a border passing themselves in
add (“border”, border);
}
border.add(c);
}
}

In this case, in the BasePage class, I’ve put a protected method – getBorder() – so that the implementing page can decide which border to create.

While this set up works, somehow this seems smelly.

OTOH, if I look at wicket itself, seems like this mutual knowledge is rampant (Page class has a getComponent) and Component classes have a getPage() – however, the coupling is lower.

Any suggestions?

Well, i was trying to solve that problem since a week

I’m waiting for new post/presentation about Oriented Object Concept !

Good luck, and thank you !

class Character {

private final Client client;
private final CharStatus status;

// other fields…

public Character(Client _client, CharStatus _status);

}

public CharStatus {

private final Client client;
private int lifePoints;

public CharStatus(Client _client, int _lifePoint);

public void fireStatusUpdated(String _property, int _old, int _new); // Using Observer/Observable pattern

}

And finally, class “C” :
class Client {

public void onStatusUpdate(Client _client, String _property, int _old, int _new);

}

The issue was I wanted my collaborator class to send an update to my “super class” Character. Then, my Character send an update to the Client, then Client send data throw network…

In fact, collaborator classes have to send an update directely to the client…
Circular dependency avoided

Sorry for posting !
Thomas.

Ps : I love your job, I think DI is the key for keeping a clean code.
Ps2 : Sorry for ma poor English =) I’m French, and French reputation is to be bad at English ^^

I have a question about circular dependency…

I have a class Character who is responsible for many and many things : appearance (face, hair…), status (lifepoints…), stats (attack power, defense power…) and i’m stuck !
Look at an example :

class Character {
private CharAppearance ca;
private CharStatus status;
private CharStats stats;

public Character(CharAppearance _ca, CharStatus _status, CharStats _stats) {
//
}

}

Example of CharStatus :
class CharStatus {

private int lifePoints;
private Character owner;

public CharStatus(Character _owner, int _lifePoint) // Here.. circular dependency !

}

How to solve that issue ? Should I remove the owner of CharStatus ?
Thanks a lot
Thomas.

here is how I would attack you problem.

class Database() implements DB;
class Logger(Database db);
class LoggingDatabase(Logger log, Database db) implements DB;

DB rawDatabase = new Database();
Logger log = new Logger(rowDatabase);
DB dbForTheRestoOfApplication = new LoggingDatabase(log, rowDatabase);

There you go. LoggingDatabase logs the commands as they go by and than delegate it on a class of the same interface. This is know as chain of responsibility and it is my favorite design pattern.