Java Generics GetThis Trick Explanation

Question

I am reading about Java Generics and I came across this topic where I am a bit confused.

From : http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#FAQ205

public 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;
     parent.children.add(this);  // error: incompatible types
   }
   public N getParent() {
     return parent;
   }
   public List<N> getChildren() {
     return children;
   }
 }

public class SpecialNode extends Node<SpecialNode> {
   public SpecialNode(SpecialNode parent) {
     super(parent);
   }
}

Scrolling lower a couple of screens...

public abstract class Node <N extends Node<N>>  {
   ...
   protected Node(N parent) {
     this.parent = parent;
     parent.children.add( (N)this ); // warning: unchecked cast
   }
   ...
 }

Casts whose target type is a type parameter cannot be verified at runtime and lead to an unchecked warning. This unsafe cast introduces the potential for unexpected ClassCastException s and is best avoided.

Could someone give me an example where the above code throws a ClassCastException ?

Thanks.

Answer 1

First code sample

In the first code sample, there is a compile-error. You can verify it yourself in your IDE.

Mine says : The method add(N) in the type List<N> is not applicable for the arguments (Node<N>)

The problem is that N is a subtype of Node. The List of N might be a list of StupidNode, where StupidNode is a subclass of Node. But the current instance might not be a StupidNode, it could be a different subclass of Node, so adding it could be wrong.

---

Second code sample

Now the second code sample is one where the developer, annoyed by the compile-time error that he doesn't understand, believes the compiler is wrong and try to force a cast. Such a cast make the code compile, but could break at runtime in the same conditions (as explained higher).

Therefore, the compiler issues a warning, to help you understand that something could be wrong.

---

Sample problem

For both previous code samples, the problem could happen if the calling code writes (for two subclasses NodeA and NodeB of Node):

Node<NodeA> root = new NodeA<NodeA>(null); 
// code needs a change, to be able to handle the root, that has no parent
// The line with parent.children will crash with a NullPointerException
Node<NodeB> child = new NodeB<NodeB>(root);

On the second line, the code that will run in the constructor of Node will interpret like (replacing the format parameter N by the current parameter NodeB):

public abstract class Node <NodeB>  {
   private final List<NodeB> children = new ArrayList<NodeB>();
   private final NodeB parent;

   protected Node(NodeB parent) {
     this.parent = parent;
     parent.children.add(this);  // error: incompatible types
   }
  // ...
 }

As you can see, the second line of the caller will pass a NodeA instance, while the constructor of Node expects a NodeB! Hence the error...

---

UPDATE as asked by comment : sample code for subclasses NodeA (or NodeB).

public class NodeA extends Node<NodeA>  {
   public NodeA(NodeA parent) {
     super(parent);
   }
}