Help with "parameter type not within its bounds"

Question

Can someone please explain the behavior I'm seeing below?
public class Node<ChildType extends Node<ChildType>>
public class SubNode<ChildType extends SubNode<ChildType>> extends Node<ChildType>
  public static void main(String[] args)
    Node<Node> myNode; // ERROR: Node not within its bounds
    Node<SubNode> myNode2; // ERROR: SubNode not within its bounds
    Node<? extends Node> myNode3; // Works
    Node<? extends Node> myNode4 = new Node<Node>(); // ERROR: Node not within its bounds
    Node<? extends Node> myNode4 = new Node<SubNode>(); // ERROR: SubNode not within its bounds
}How do I get it to work the way I expect? I want to construct a Node with a parameterized children type so that if you construct a Node, the children would also be of type Node. If you construct a SubNode, the children would also be of type SubNode, etc.
Thank you,
Gili

Answer 1

I would make the "concrete" subclasses public:
import java.util.Collection;
import java.util.Collections;
public interface Node<C extends Node<C>> {
    Collection<C> getChildren();
public interface SubNode<C extends SubNode<C>> extends Node<C> {
    C getParent();
abstract class DefaultNode<C extends Node<C>> {
    Collection<C> children;
    DefaultNode(Collection<C> children) {
     this.children = Collections.unmodifiableCollection(children);
    public Collection<C> getChildren() {
     return children;
abstract class DefaultSubNode<C extends SubNode<C>> extends DefaultNode<C> {
    C parent;
    DefaultSubNode(Collection<C> children, C parent) {
     super(children);
     this.parent = parent;
    public C getParent() {
     return parent;
public class ConcreteNode extends DefaultNode<ConcreteNode>
                implements Node<ConcreteNode> {
    public ConcreteNode(Collection<ConcreteNode> children) {
     super(children);
public class ConcreteSubNode extends DefaultSubNode<ConcreteSubNode>
                   implements SubNode<ConcreteSubNode> {
    public ConcreteSubNode(Collection<ConcreteSubNode> children, ConcreteSubNode parent) {
     super(children, parent);
}