在java实现单链表中，为什么链表的最后一个元素没有打印出来？

Question

public class Linked_List <E>{
    public static class Node<E>{
        private E element;
        private Node<E> next;
        public Node(E e,Node<E> n) {
        element=e;
        next=n;
    }
    public E getElement() {
        return element;
    }
    public Node<E> getNext() {
        return next;
    }
    public void setNext(Node<E> n) {
        next=n;
    }
}

public Node<E> head=null;
public Node<E> tail=null;
public int size=0;

public Linked_List() {}

public int size() {
    return size;
}

public boolean isEmpty() {
    return size==0;
}
public void addFirst(E e) {
    head=new Node<>(e,head);
    if(size==0)
        head=tail;
    size++;
}

public void addLast(E e) {
    Node<E> newest =new Node<>(e,null);
    if(isEmpty())
        head=newest;
    else
        tail.setNext(newest);
    tail=newest;
    size++;
}
public void show() {

    Node<E> n=head;
    if(size==0) {
        System.out.println("No elements to print");
        System.exit(0);
    }
    while(n.next!=null) {
        System.out.println(n.element);
        n=n.next;
    }
    System.out.println(n.element);
}

public static void main(String[] args) {
    Linked_List<Integer> list = new Linked_List<Integer>();

    list.addFirst(10);
    list.addFirst(11);
    list.addFirst(12);

    list.show();

}
}

In show() method when the while reaches the last element of the list, it exits so the element doesn't get printed. Hence the last print statement in the show method. I have added three elements into the list but when I execute the show() method only the first two elements that is 12 and 11 get printed. What is it that i am missing? Thanks.

Answer 1

How about here. This should say tail = head ;

    public void addFirst(E e) {
        head = new Node<>(e, head);
        if (size == 0) {
            head = tail;
        }
        size++;
    }

Answer 2

addFisrt() change to:

public void addFirst(E e) {
    head = new Node<>(e, head);
    size++;
}

you can debug addFirst(), the first element never add to LinkedList.

Answer 3

The problem is in the while loop condition:

while(n.next!=null) {
        System.out.println(n.element);
        n=n.next;
    }

When the list reaches element n-1 it will print the element and then n will become n=n.next which is the last element. But now n being the last element his n.next will be null therefore while will break since the condition is not met anymore.

Answer 4

You answered your own question:

In show() method when the while reaches the last element of the list, it exits so the element doesn't get printed.

This code:

while(n.next!=null) {
    System.out.println(n.element);
    n=n.next;
}

says "while it's not the last element, print it". ie, the code explicitly does not want to print the last element.

I am here assuming that you're not including the final print statement in your problem description - that you added the final print to work around the while-loop problem.

You need:

while (n != null) {
    System.out.println(n.element);
    n = n.next;
}

which says 'while it is an actual element, print it'.