Re: Automatic linking of related objects in constructor

From:

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Newsgroups:

comp.lang.java.programmer

Date:

Wed, 29 Jun 2011 09:17:29 -0700

Message-ID:

<iufj6q$tmv$1@dont-email.me>

On 6/29/2011 2:56 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.

Yes, imo this is correct. It seems that sometimes you have to do it
(constructing Swing components) but in general it's a bad idea.

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 like the factory pattern best here

class A {

   private ArrayList<B> list;

   private A() {}

   /** Factory for constructing new A's */
   public static A newInstance( B... bs ) {
     A a = new A();
     a.list = new ArrayList<B>();
     a.list.addAll( bs );
     return a;
   }
}