java链表应用--基于链表实现队列详解（尾指针操作）

package Queue;

public interface Queue<E> {
  //获取队列中元素个数
  int getSize();

  //队列中元素是否为空
  boolean isEmpty();

  //入队列
  void enqueue(E e);

  //出队列
  public E dequeue();

  //获取队首元素
  public E getFront();
}

package Queue;

public class LinkedListQueue<E> implements Queue<E> {

  //将Node节点设计成私有的类中类
  private class Node<E> {
    public E e;
    public Node next;

    //两个参数的构造函数
    public Node(E e, Node next) {
      this.e = e;
      this.next = next;
    }

    //一个参数的构造函数
    public Node(E e) {
      this.e = e;
      this.next = null;
    }

    //无参构造函数
    public Node() {
      this(null, null);
    }

    @Override
    public String toString() {
      return e.toString();
    }
  }

  private Node<E> head, tail;

  private int size;


  //显示初始化
  public LinkedListQueue() {
    head = null;
    tail = null;
    size = 0;
  }

  //获取队列中节点个数
  @Override
  public int getSize() {
    return size;
  }

  //队列中是否为空
  @Override
  public boolean isEmpty() {
    return size == 0;
  }

  //链表尾部进队操作
  @Override
  public void enqueue(E e) {
    if (tail == null) {
      tail = new Node(e);
      head = tail;
    } else {
      tail.next = new Node(e);
      tail = tail.next;
    }
    size++;
  }

  //链表头部出队操作
  @Override
  public E dequeue() {
    if (isEmpty()) {
      throw new IllegalArgumentException("链表为空");
    }

    Node<E> retNode = head;
    head = head.next;
    retNode.next = null;
    if (head == null) {//当链表只有一个元素时
      tail = null;
    }

    size--;
    return retNode.e;
  }

  //获取队首元素
  @Override
  public E getFront() {
    if (isEmpty()) {
      throw new IllegalArgumentException("链表为空");
    }
    return head.e;
  }


  //为了便于测试，重写object类toString()方法
  @Override
  public String toString() {
    StringBuilder res = new StringBuilder();
    res.append("Queue: front ");
    Node<E> cur = head;
    while (cur != null) {
      res.append(cur + "->");
      cur = cur.next;
    }
    res.append("NULL tail");
    return res.toString();
  }


}

 //测试用例
  public static void main(String[] args) {
    LinkedListQueue<Integer> queue = new LinkedListQueue<Integer>();
    for (int i = 0; i < 10; i++) {
      queue.enqueue(i);
      System.out.println(queue);

      if (i % 3 == 2) {//每添加3个元素出队列一个
        queue.dequeue();
        System.out.println(queue);
      }

    }

  }