StackOverflow2013

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copied!<p>There is a cycle in the reference graph. A references B and B references A. The garbage collector will detect cycles and see when there are no external references to A and B, and will then collect both.</p> <p>Attempting to use the finaliser here is wrong. If B is being destroyed, the reference to A is also being removed.</p> <hr> <p>The statement: "Assume that the A instance will be shared with some other objects as well and will outlive the B instance." is wrong. The only way that will happen is if the listener is explicitly removed from somewhere other than a finalizer. If references to A are passed around, that will imply a reference to B, and B will not be garbage collected because there are external references to the A-B cycle.</p> <hr> <p>Further update:</p> <p>If you want to break the cycle and not require B to explicitly remove the listener, you can use a WeakReference. Something like this:</p> <pre><code>class A { void addListener(Listener obj); void removeListener(Listener obj); } class B { private static class InnerListener implements Listener { private WeakReference m_owner; private WeakReference m_source; InnerListener(B owner, A source) { m_owner = new WeakReference(owner); m_source = new WeakReference(source); } void listen() { // Handling reentrancy on this function left as an excercise. B b = (B)m_owner.get(); if (b == null) { if (m_source != null) { A a = (A) m_source.get(); if (a != null) { a.removeListener(this); m_source = null; } } return; } ... } } private A a; B() { a = new A(); a.addListener(new InnerListener(this, a)); } } </code></pre> <p>Could be further generalised if needed across multiple classes.</p>

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