Re: Automatic linking of related objects in constructor
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