Java-自定义迭代器无法跟踪自定义链接列表的头部

Question

The classes are not complete, but here's what I have so far and I expected the test below to pass.

public class LinkedList<T> extends AbstractSequentialList<T> {
  private Node<T> head;

  @Override
  public boolean add(T element) {
    if(head == null) {
      head = new Node(element);
    }
    return true;
  }

  @Override
  public ListIterator<T> listIterator(int index) {
    return new LinkedListIterator<>();
  }

  @Override
  public int size() {
    return 0;
  }

  private class LinkedListIterator<T> implements ListIterator<T> {
    private Node<T> current;

    public LinkedListIterator() {
        current = (Node<T>) head;
    }

    @Override
    public boolean hasNext() {
        return (current != null && current.getNext() != null)? true : false;
    }

    @Override
    public T next() {
        return null;
    }
  }
}

Here is the Node class.

public class Node<T> {

  private T value;
  private Node next;

  public Node(T value) {
    this.value = value;
  }

  public Node(T value, Node next) {
    this.value = value;
    this.next = next;
  }

  public T getValue() {
    return value;
  }

  public Node getNext() {
      return next;
  }

  public void setNext(Node next) {
      this.next = next;
  }
}

My iterator test is like this.

LinkedList<String> list;
ListIterator<String> iterator;

@Before
public void setUp() throws Exception {
    list = new LinkedList<>();
    iterator = list.listIterator();
}

@Test
public void testHasNext() throws Exception {
    assertThat(iterator.hasNext(), is(false));

    list.add("Hello World");
    assertThat(iterator.hasNext(), is(true));
}

However, I'm failing on the second assertion. My issue is that the "current" pointer in the iterator is always null even though I'm setting it to the head of the enclosing LinkedList class. How can I fix this? Thanks.

Answer 1

It looks the value of current is set inside the constructor of LinkedListIterator.

It hasn't been updated after you have added an element to the list. This seems to your problem here.

Answer 2

What is wrong is your test, IMO.

You shouldn't expect the iterator to point to the first element if the first element has been added after the iterator has been constructed.

Now, why does your iterator work this way? Because Java is pass-by-value. When you construct an iterator, the iterator receives a copy of the reference to the first node of the list. And at this time, this reference is null, because you haven't added any node yet.

If you really want the iterator to "see" the first node of the list even after it has been constructed, then the iterator needs to get the first node of the list in hasNext() , not in the constructor.