将队列实现为列表有什么问题？

Question

I started with creating the variables head and tail of the queue, which are elements of a list themselves of course, and I create an empty list in the constructor by writing head = tail = null . In enq , I create a new element of the list called help , and it is supposed to save the new element that I want to put at the end of the list (since it is a queue). Of course the tail of the queue is always the element that we just put at the end of the list before. Then I check whether head is empty or not (either because the list itself is empty or because we deleted the first element of it, which should be always identical with head ), and, if necessary, I define head as help too. Then help is supposed to show to a new empty element of the list.

The problem is the following: When I test my program, it stops after the very first delete and tells me that the list is already empty.

I guess the mistake lies within the method deq . I want to delete the first element by writing head = head.next , but we defined head as help before, thus, head.next is identical with help.next , which is identical with null , so the boolean -method returns that the list is empty, which stops the programm.

I might have to change the the "direction" of help.next , but I don't see how. How can make this whole thing work?

Creating the elements of the list:

public class Entry {
  Object content;
  Entry next;
}

Implementation of List:

public class QueueList implements List {

  private Entry head;
  private Entry tail;

  public QueueList() {
    head = tail = null;
  }

  public boolean empty() {
    return head == null;
  }

  public void enq(Object x) {
    Entry help = new Entry();
    help.content = x;
    tail = help;

    if (head == null) {
      head = help;
    }

    help.next = null;
  }

  public Object front() {
    return head.content;
  }

  public void deq() {
    head = head.next;
  }
}

Answer 1

  public void enq(Object x) {
    Entry help = new Entry();
    help.content = x;
    **tail = help;**

    if (head == null) {
      head = help;
    }

    help.next = null;
  }

There always will be only one element in the queue at any time.

This can be solved using the below enq() method

public void enq(Object x) {
        Entry help = new Entry(x);

        if (head == null) {

            head = help;

            tail = head;

        } else {
            if (head == tail) {

                head.next = help;
                tail = help;



            } else {

                this.tail.next = help;
                this.tail = help;
            }
        }

    }