Breadth First Binary Tree Search using TreeNodes Extending Comparable

Question

I'm having issues implementing my breadth first traversal for my binary tree. I keep getting a class cast exception at these two lines:

        if(node.left != null) queue.offer(node.left);
        if(node.right != null) queue.offer(node.right);

The exception says: "TreeNode cannot be cast to java.lang.Comparable" I need the extends Comparable for other methods this class, any suggestions on how to fix this? Thanks!

public class TreeNode<E extends Comparable<E>> {

private TreeNode<E> left;
private TreeNode<E> right;
private E value;


public TreeNode(E value) 
{
    this.value = value;
}

    public void breadthFirstTraversal(TreeNode<E> root, ArrayList<E> breadthList)
{
    Queue<TreeNode<E>> queue = new PriorityQueue<TreeNode<E>>();
    if (root == null)
        return;
    queue.clear();
    queue.offer(root);
    while(!queue.isEmpty())
    {
        TreeNode<E> node = queue.remove();
        breadthList.add(node.value);
        if(node.left != null) queue.offer(node.left);
        if(node.right != null) queue.offer(node.right);
    }
}

Answer 1

You have:

public class TreeNode<E extends Comparable<E>>

This only says that the E in TreeNode<E> should implement Comparable, not that the TreeNode itself should. For that you would need to make TreeNode implement Comparable in the usual fashion:

public class TreeNode<E extends Comparable<E>> implements Comparable<TreeNode<E>> 

And add the appropriate overrides.