get the parent (or container?) of an object?

[Joe Vahabzadeh]

Ok, first off, I'm probably using wildly inaccurate terminology in the subject line.

Let's say I have a class called Office, and classes that work with it called Chair, Desk, and Table, such that, in bad pseudocode:

public class Office
{
     public Chair c = new Chair();
     public Desk d = new Desk();
     public Table t = new Table();
}

Now, in the program there may be several instances of Office.

Let's say I've got some code in my Desk class that needs to find out something about the Chair that is in the same Office instance. How do I get a reference to the particular instance of Office that the Desk is in?

I have a feeling this is something fairly trivial, but I can't recall how to do this, if it can be done.

[Rob Spoor]

Have a member field that is a reference to its parent, and set it in the constructor:

public class Chair /* any extension or implementation here */ {
    private Office office;
 
    public Chair(Office o) {
        // any checks on o
        office = o;
        // any other code
    }
 
    public Office getOffice() {
        return office;
    }
}

Of course, you could perhaps use setOffice(Office o) or moveTo(Office o) methods to change the office member.

Of course, it would be best to have some interface or abstract base class for Office, let's say Space, so you can move chairs etc to different kinds of spaces (office, bedroom, etc) without much code change. And you can do the same for Chair, Table, etc. If you make that an abstract class (called Furniture) you can put the moving code into that one abstract base class.

[Joe Vahabzadeh]

Ah, that's the rub.... it's existing code, and I'm not allowed to do that.

So there's no real way given the Desk and absolutely no other information, to get the Office instance that it's inside of?

Sorry, forgot to put that restriction in my original question.

[Ernest Friedman-Hill]

Have you got a collection of all the Office objects? You could look at each one and check its Desk(s).

[Paul Clapham]

Nope. An Office object just contains a reference to a Chair object, and there's nothing in Java that lets you find out what other objects contain references to a particular object.

So, if this is now a requirement (clearly it wasn't before) then the data model will have to change to accomodate it. As per Rob's suggestion perhaps.

[Rob Spoor]

If you really cannot change the current code you can create auxiliary classes that store the relations. Although it's bad practice, it might be the only way.

These classes then keep your connections between spaces and furniture.

[Joe Vahabzadeh]

Thanks for all the suggestions. So, essentially if Desk x is a Desk that is in Office y, there's no way, given x alone, to find y.

On the other hand, with the problem that I was working on, there did turn out to be a reference to the Office in the first place somewhere.

Buried, of course! Ah, the joys of working on pre-existing code!

Anyway, thanks for the advice and information.

[Stuart Ash]

Originally posted by Rob Spoor:
If you really cannot change the current code you can create auxiliary classes that store the relations. Although it's bad practice, it might be the only way.

These classes then keep your connections between spaces and furniture.

Yes, let's say a class called Furniture which has references to all the furniture items.