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From: Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Automatic linking of related objects in constructor
Date: Wed, 29 Jun 2011 08:29:00 -0400
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On 6/29/2011 5:56 AM, Qu0ll wrote:
> Suppose you have class A which contains a list of objects of class B and
> that the constructor for B takes in a reference to the A object which is
> to include it in said list. Is it safe to make a call to A's method
> addB(B b) in that B constructor passing in "this"? I have heard that
> it's bad practice to "leak" a reference to an object from within its own
> constructor because it may be in an invalid state.

     I guess you mean

	class B {
	    public B(A a) {
	        ...
	        a.addB(this);
	    }
	    ...
	}

It's not a wonderful idea, but even if it's not safe it looks sort
of safe-ish, right?  Ah, but code changes, and lo! one of the
changes introduces a subclass:

	class C extends B {
	    public C(A a) {
	        super(a);
	        ...
	    }
	}

Okay, still not *too* bad -- or is it?  Let's expand that last
set of ellipses:

	class C extends B {
	    public C(A a) {
	        super(a);
	        if (phaseOfTheMoon() != Moon.FULL)
	            throw new IllegalStateException();
	        ...
	    }
	}

Yikes!  Now if C's constructor throws up, the A instance is left
holding a reference to something that appears to be a C in B's
clothing, but is in fact nothing sensible at all!

> If not, how else can I automatically add the B object to the list in A
> without forcing the client programmer to explicitly call addB() given
> that they have already passed in the B as an argument?

     (I assume you meant "A as an argument.")  Consider using a
factory method:

	class B {
	    private B(A a) {
	        // so clients can't do "new B(a)"
	        ...
	    }
	    public static B instanceOf(A a) {
	        B b = new B(a);
	        // B's constructor is now completely finished
	        a.addB(b);
	        return b;
	    }
	}

-- 
Eric Sosman
esosman@ieee-dot-org.invalid