generic type scope does not apply to inner class?

Question

I have the following code where type T extends Comparable, but I get a compile error when I try to do

root.node.compareTo(min).

Saying compareTo does not apply. It seems generic type scope does not apply to inner class?

public class Tree<T extends Comparable> {
    public class Node<T>{
        private T node;
        private Node<T> left;
        private Node<T> right;
        public Node(T node, Node<T> left, Node<T> right) {
            super();
            this.node = node;
            this.left = left;
            this.right = right;
        }       
    }
    public boolean isBalanced(Node<T> root){
        return isBalanced(root, Integer.MIN, Integer.MAX);
    }
    private boolean isBalanced(Node<T> root, T min, T max){
        if(root == null){
            return true;
        }
        if(root.node.compareTo(min) < 0 || root.node.compareTo(max) > 0){
            return false;
        }
        return isBalanced(root.left, min, root.node) || isBalanced(root.right, root.node, max);
    }
}

Answer 1

When you say public class Node<T>, you're shadowing the <T> from the outer class. You should either remove the <T> from the inner class declaration or, better, make the inner class a static nested class (since there's no inherent reason the node needs to know about the tree it's attached to).

(Note: Also use <T extends Comparable<T>>.)