Question

these are my fields:

public class BSTSet <E> extends AbstractSet <E> {

    // Data fields
    private BSTNode root;
    private int count = 0;
    private Comparator<E> comp;   // default comparator

    /** Private class for the nodes.
     *  Has public fields so methods in BSTSet can access fields directly. 
     */
    private class BSTNode {

        // Data fields

        public E value;
        public BSTNode left = null;
        public BSTNode right = null;

        // Constructor

        public BSTNode(E v) {
            value = v;
        }

        //creates a method called contains so that i can call it later on for my find method
        public boolean contains(Object item) {
            return contains(item);//root.value.equals(item);
        }

        public int height() {
            return height();
        }

    }
    // Constructors - can either use a default comparator or provide one
    public BSTSet() {
        comp = new ComparableComparator();      // Declared below
    }

    public BSTSet(Comparator <E> c) {
        comp = c;
    }
}

and this is what i am trying to complete:

private class BSTSetIterator implements Iterator<E> {

    private Stack<BSTNode> stack = new Stack<BSTNode>();
    private BSTNode current = root;

    public BSTSetIterator(BSTNode root) {

         return new BSTSetIterator();

    }

    public boolean hasNext() {

        boolean hasNext = false;
        hasNext = !stack.isEmpty() || current != null;
        return hasNext;

    }

    public E next() {

        BSTNode next = null;

        while (current != null) {
            stack.push(current);
            current = current.left;
        }
        next = stack.pop();
        current = next.right;

        return next;

    }

    public void remove() {
        throw new UnsupportedOperationException();
    }
}
// Comparator for comparable 

private class ComparableComparator implements Comparator<E> {
    public int compare(E ob1, E ob2) {
        return ((Comparable)ob1).compareTo(ob2);
    }
}

So far the code fails at lines return new BSTSetIterator(); and return next;. For return next it says that it is the wrong data type to return. How would I go about fixing these methods so that I can iterate through a BST using a Stack?

Answer 1

BSTSetIterator();

This doesn’t work, because your constructor expects a root and you didn’t pass that parameter. If you have a BSTSet object called ‘tree’, and you want to create a new iterator, then you should create the iterator this way:

BSTSetIterator iterator = new BSTSetIterator(tree.getRoot());

However, you don’t have a getter in your BSTSet class and your root is private. Don’t worry, the solution for that problem is to create a public getter inside your BSTSetIterator class, like this:

public BSTNode getRoot()
{
    return this.root;
}

Constructors don’t return values, this is incorrect:

 public BSTSetIterator(BSTNode root) {
         return new BSTSetIterator();
    }

Instead, write your construtor this way:

public BSTSetIterator(BSTNode root)
{
    this.current = root;
}

Also, this definition is incorrect, because root is out of reach:

private BSTNode current = root;

You should have this instead:

private BSTNode current;

As for your other problem,

BSTNode next = null;

means that your variable called ‘next’ is of BSTNode type.

public E next()

means that your method called next is of E type. as E and BSTNode is not the same, your return:

return next;

is incorrect. I could give you more help, but I have realized you are learning now the language and it’s better to let you explore yourself the technology and programming in general, because this way you will become quicker. “Give a man a fish, and you feed him for a day. Teach a man how to fish, and you feed him for a lifetime.”