How do I recover the actual type of the this object in a class hierarchy?

Question

Hi,
This is code snippet from Angelica Langer's FAQ:
abstract class Node <N extends Node<N>>  {
     private final List<N> children = new ArrayList<N>();
   private final N parent;
   protected Node(N parent) {
        this.parent = parent;
        if(parent!=null)
           parent.children.add(this);  // error: incompatible types
   public N getParent() {
     return parent;
   public List<N> getChildren() {
     return children;
}There are 3 different approaches available in the FAQ to solve this problem.
My question is:
If I write:
parent.children.add((N)this);  // cast added here
...as described at the end of this topic in the FAQ, am I safe here (despite
the compilers warning) ? In my opinion, although I understand that
compiler needs to issue a warning, it is actually 100% safe .... or is it not ?
Or let's ask more precise question: can it throw a ClassCastException at
runtime ever ? If so, can you give any example ?
Thanks,
Adrian

Answer 1

You might not get a CCE within the Node class (because of erasure), but maybe on accessing the children, e.g., in a loop, where the cast will be created by the compiler, based on the type information you give to it.
class A extends Node<A> {
  public A(A parent) { super(parent); }
class B extends Node<A> {
  public B(A parent) { super(parent); } // This will add a B as child to an A!
A a = ...;
B b = new B(a);
for (A anA : b.getChildren()) { ... }Creating an instance of B will add it as child to a List<A>, which will most probably result in a CCE when looping on the list like shown above, which is type-safe from the compiler point of view.