How to implement a breadth first traversal with no parameters?

I am writing this method in a class with the following variables initialized:

public class NonEmpty<T extends Comparable<T>> implements BST<T> {
    private T _element;
    private BST<T> _left;
    private BST<T> _right;

    public NonEmpty(T element) {
        _left = new Empty<T>();
        _right = new Empty<T>();
        _element = element;
    }
...

My method so far is as follows:

public void BFT() { 
        if (_element == null) {
            return;
        }
        Queue<TreeNode> queue = new LinkedList();
        queue.add(_element); // Error here, it says my required type is T

        while (!queue.isEmpty()) {
            TreeNode node = queue.remove();
            System.out.print(_element + " "); // print node data

            if (getLeft() != null) {
                queue.add(node.getLeft()); // Error here, it says it cannot resolve the method though it is defined else where
            }

            if (getRight() != null) {
                queue.add(node.getRight()); // Error here, it says it cannot resolve the method though it is defined else where
            }
        }

    }

Not sure why I'm getting these errors, would appreciate your help