基于链表实现Java 自定义Queue队列

与栈一样，我们也可以借助单链表来实现队列ADT。同样地，出于效率方面的考虑，我们将以单链表的首（末）节点作为队列的首（末）节点??这样，可以回避单链表在尾部进行删除操作时效率低下的缺陷。此外，还需要两个实例变量分别指示表的首、末节点。

java代码如下：

QueueList:

package com.list.queue;
import java.util.Arrays;

import com.list.stack.Node;
import com.list.stack.ExceptionStackEmpty;

/**
 * 
 * @author gannyee
 *
 */
public class QueueList {

    //Declared head node
    private static Node head = new Node();
    //Declared rear node
    private static Node rear = new Node();
    //Declared size
    private static int size;
    //queue list
    private static int length;
    //Constructor initialize head, rear, size
    public QueueList(){
        head = rear = null;
        size = length = 0;
    }

    //Get size
    public int getSize(){
        return size;
    }

    //Get length
        public int length(){
            return length;
        }

    //Is empty
    public boolean isEmpty(){
        return size == 0;
    }

    //Enqueue element
    public void enqueue(Object element){
        Node newNode = new Node(element,head);
        if(getSize() == 0){
            head = newNode;
            rear = newNode;
        }
        head = newNode;
        size ++;
        length = size;
    }

    //Dequeue element
    public Object dequeue() throws ExceptionStackEmpty{
        if(isEmpty())
            throw new ExceptionStackEmpty("Queue is empty!");
        Node travelNode = new Node();
        travelNode = head;
        Object elementRear;
        while(travelNode != null){
            if(travelNode.getNext() == rear)
                break;
            travelNode = travelNode.getNext();
        }
        elementRear = rear.getElement();
        rear = travelNode;
        size --;
        return elementRear;
    }

    //Get head node but not delete
    public Object getHeadNode() throws ExceptionStackEmpty{
        if(isEmpty())
            throw new ExceptionStackEmpty("Queue is empty!");
        return head.getElement();
    }

    //Travel all node of queue
    public void getAllElement(){
        Node travelTop;
        travelTop = head;
        Object[] array = new Object[getSize()];

        for(int i = 0;i < array.length;i ++ ){
            array[i] = travelTop.getElement();
            travelTop = travelTop.getNext();
        }
        System.out.println("Get all elemnt: " + Arrays.toString(array));
    }

}

QueueListTest :

package com.list.queue;

import com.list.stack.ExceptionStackEmpty;

public class QueueListTest {

    public static void main(String[] args) throws ExceptionStackEmpty {
        // TODO Auto-generated method stub

        //Declared QueueList example
        QueueList ql = new QueueList();

        System.out.println("Size is: " + ql.getSize());
        System.out.println("Is empty? " + ql.isEmpty());

        ql.enqueue(12);
        ql.enqueue(32);
        ql.enqueue(35);
        ql.enqueue(12);
        ql.enqueue(34);
        ql.enqueue(45);
        ql.enqueue(17);
        ql.enqueue(2);
        ql.enqueue(15);

        System.out.println("Size is: " + ql.getSize());
        System.out.println("Is empty? " + ql.isEmpty());
        ql.getAllElement();


        for(int i = 0;i < ql.length();i ++)
            System.out.println(ql.dequeue());


        System.out.println("Size is: " + ql.getSize());
        System.out.println("Is empty? " + ql.isEmpty());
    }

}

测试结果：

Size is: 0
Is empty? true
Size is: 9
Is empty? false
Get all elemnt: [15, 2, 17, 45, 34, 12, 35, 32, 12]
12
32
35
12
34
45
17
2
15
Size is: 0
Is empty? true

与基于单链表实现的栈结构同理，如上实现的各个队列 ADT 方法都可以在 O(1)时间内完成。而且，这里同样无需限定队列的最大规模。当然，受单链表结构的限制，我们仍然需要对各种特殊情况（如队空时）专门处理。