LinkedList的Java实现输出null

Question

I'm having a bit of trouble here, I'm implementing my own Linked List class that does what you expect, it adds elements to a list and using a ToString method, outputs them. For some reason, it adds elements to the list fine but when it's time to output the list, it adds a null element to the front and ignores printing the last element. Here is my main class code:

public class MyList {
    public static void main(String[] args) {
        List<Integer> list = new LinkedList<>();
        list.add(1);                 // [1]
        list.add(2);                 // [1 2]
        list.add(3);                 // [1 2 3]
        System.out.println(list);
    }
}

And here's the add and ToString method:

public class LinkedList<E>  {
private Node<E> first, last;
private int size = 0;

// new empty list constructor.
public LinkedList() {
    first = last = new Node<>(null, null);
}

public void add(E e) {
    last.next = new Node<>(e, null);
    last = last.next;
    ++size;
}

public void addFirst(E e) {
    Node<E> n = new Node<>(e, null);
    n.next = first.next;
    first = n;
    ++size;

}

public String toString() {
    try {
        if (first != null) {
            Node<E> n = first;
            String s = "[ ";

            while (n.next != null) {
                s = s + n.data + " ";
                n = n.next;
            }

            return s + "]";
        }
    } catch (NoSuchElementException e) {
        //return "List is empty!";
    }

    return "List is empty!";
}

And when trying to compile the above, I get this output (when it should be [ 1 2 3 ] ):

> [ null 1 2 ]

Any help would be greatly appreciated. Thank you.

Answer 1

This line:

last.next = new Node<>(e, null);

should throw a NullPointerException when you add the first element to the list, because first and last should be null when the list is empty. The fact that it's apparently not erroring out on you means that last must be pointing to a valid Node object before you've added anything to your list. You don't have all of your code in your post, but I'm pretty confident that you're creating a new Node and pointing first and last to it when you call your list's constructor, which means that your list will start with an "empty" node.

What you should be doing is starting with both first and last equal to null . Then do something like this in your add() method:

public void add(E e) {
    Node newNode = new Node<>(e, null);
    if(first == null) {
        first = newNode;
        last = newNode;
    } else {
        last.next = newNode;
        last = last.next;  // Or just last = newNode;
    }
    ++size;
}

Also, you need to work on your toString() method a bit, because this line:

while (n.next != null) {

will cause your loop to bypass the last element in the list (since its correct for the list element's next to be null).