二叉树迭代器无法正常运行以进行遍历

Question

我正在尝试使用迭代器来完成对链接的二叉树的有序遍历。但是，当我从树上的迭代器上调用iterator（）。next（）时，它总是抛出NoSuchElementException异常。为简单起见，我将使用一棵只有一个单节点的树，该树称为ir singleNodeTree。我不知道为什么迭代器没有按预期方式遍历树，而我在哪里做错了？

BinaryTree.java

/**
     * Returns an iterator over the elements in this tree using the
     * iteratorInOrder method
     *
     * @return an in order iterator over this binary tree
     */
    public Iterator<T> iterator()
    {
        return iteratorInOrder();
    }

    /**
     * Performs an inorder traversal on this binary tree by calling an
     * overloaded, recursive inorder method that starts with
     * the root.
     *
     * @return an in order iterator over this binary tree
     */
    public Iterator<T> iteratorInOrder()
    {
        ArrayUnorderedList<T> tempList = new ArrayUnorderedList<T>();
        inOrder(root, tempList);

        return new TreeIterator(tempList.iterator());
    }

    /**
     * Performs a recursive inorder traversal.
     *
     * @param node the node to be used as the root for this traversal
     * @param tempList the temporary list for use in this traversal
     */
    protected void inOrder(BinaryTreeNode<T> node,
            ArrayUnorderedList<T> tempList)
    {
        if (node != null)
        {
            inOrder(node.getLeft(), tempList);
            tempList.addToRear(node.getElement());
            inOrder(node.getRight(), tempList);
        }
    }

/**
     * Inner class to represent an iterator over the elements of this tree
     */
    private class TreeIterator implements Iterator<T>
    {
        private int expectedModCount;
        private Iterator<T> iter;

        /**
         * Sets up this iterator using the specified iterator.
         *
         * @param iter the list iterator created by a tree traversal
         */
        public TreeIterator(Iterator<T> iter)
        {
            this.iter = iter;
            expectedModCount = modCount;
        }

        /**
         * Returns true if this iterator has at least one more element
         * to deliver in the iteration.
         *
         * @return  true if this iterator has at least one more element to deliver
         *          in the iteration
         * @throws  ConcurrentModificationException if the collection has changed
         *          while the iterator is in use
         */
        public boolean hasNext() throws ConcurrentModificationException
        {
            if (!(modCount == expectedModCount))
                throw new ConcurrentModificationException();

            return (iter.hasNext());
        }

        /**
         * Returns the next element in the iteration. If there are no
         * more elements in this iteration, a NoSuchElementException is
         * thrown.
         *
         * @return the next element in the iteration
         * @throws NoSuchElementException if the iterator is empty
         */
        public T next() throws NoSuchElementException
        {
            if (hasNext())
                return (iter.next());
            else
                throw new NoSuchElementException();
        }

        /**
         * The remove operation is not supported.
         *
         * @throws UnsupportedOperationException if the remove operation is called
         */
        public void remove()
        {
            throw new UnsupportedOperationException();
        }
    }

驱动程序。 java

public class Driver
{

 public static void main(String[] args)
 {
    //singleNodeTree is a tree with a single node element "a".
    //It has some basic method: 
    //1. getRootElement() return current element at the root
    //2. size() return size of the tree
    //3. contains(String a) reutrn true if string element a is in the tree
    BinaryTree singleNodeTree = new BinaryTree("a");
    System.out.println(singleNodeTree.getTree().getRootElement()); //return "a"
    System.out.println(singleNodeTree.getTree().size()); // return 1
    System.out.println(singleNodeTree.getTree().contains("a")); //return true

    System.out.println(singleNodeTree.getTree().iterator().hasNext());//return false
    if (singleNodeTree.getTree().iterator().next()!= null)// throws NoSuchElementException
    System.out.println("It is not empty.");  

    /* what I intend to do is the following:
    while (singleNodeTree.getTree().iterator().hasNext())
       System.out.println(singleNodeTree.getTree().iterator().next()+ " "); */

 }
}

Answer 1

异常从这里抛出：

    /**
     * Returns the next element in the iteration. If there are no
     * more elements in this iteration, a NoSuchElementException is
     * thrown.
     *
     * @return the next element in the iteration
     * @throws NoSuchElementException if the iterator is empty
     */
    public T next() throws NoSuchElementException
    {
        if (hasNext())
            return (iter.next());
        else
            throw new NoSuchElementException(); <-- This is where the Exception is thrown. 
    }

您需要将元素添加到列表中才能获得下一个元素。