In-order retrieval of elements in a binary search tree

Question

I am trying to create a binary search tree with an emphasis on recursion and am currently stuck on creating a function to return an arraylist of the elements of the the binary search tree in order.

My main problem is moreso how to implement this algorithm into a function that returns an arraylist.

Here is the beginning of the class:

public class BinarySearchTree<T> {
    private Comparator<T> comparator;
    private T data;
    private BinarySearchTree<T> left;
    private BinarySearchTree<T> right;

This is the function I current have:

public List<T> getElements() {
        List<T> list = new ArrayList<>();
        if(data != null) {
            left.getElements();
            list.add(data);
            right.getElements();
        }
        return list;
    }

Here is my insert function, which I believe is correct but if the problem is rooted here, it may help debugging:

public void insert(T element) {
        if(data == null) {
            data = element;
        }
        if(element != null && data != null) {
            if(comparator.compare(element, data) == -1) {
                if(left == null) {
                    left = new BinarySearchTree<T>(element, comparator);
                }
                else {
                    left.insert(element);
                }
            }
            if(comparator.compare(element, data) == 1) {
                if(right == null) {
                    right = new BinarySearchTree<T>(element, comparator);
                }
                else {
                    right.insert(element);
                }
            }
            if(comparator.compare(element, data) == 0) {
                return;
            }
        }

Answer 1

You need to add the results from your recursive calls to getElements to list (which, by the way, is a really bad name).

Also, you'll need to consider how to handle the cases where either left and/or right is null.