java 实现数据结构之队列

队列是一种特殊的线性表，它只允许在表的前端(front)进行删除操作，只允许在表的后端（rear）进行插入操作。 
1.队列的顺序存储结构及实现

public class SequenceQueue<T>
{
    private int DEFAULT_SIZE = 10;
    //保存数组的长度。
    private int capacity;
    //定义一个数组用于保存顺序队列的元素
    private Object[] elementData;
    //保存顺序队列中元素的当前个数
    private int front = 0;
    private int rear = 0;
    //以默认数组长度创建空顺序队列
    public SequenceQueue()
    {
        capacity = DEFAULT_SIZE;
        elementData = new Object[capacity];
    }
    //以一个初始化元素来创建顺序队列
    public SequenceQueue(T element)
    {
        this();
        elementData[0] = element;
        rear++;
    }
    /**
     * 以指定长度的数组来创建顺序队列
     * @param element 指定顺序队列中第一个元素
     * @param initSize 指定顺序队列底层数组的长度
     */
    public SequenceQueue(T element , int initSize)
    {
        this.capacity = initSize;
        elementData = new Object[capacity];
        elementData[0] = element;
        rear++;
    }
    //获取顺序队列的大小
    public int length()
    {
        return rear - front;
    }
    //插入队列
    public void add(T element)
    {
        if (rear > capacity - 1)
        {
            throw new IndexOutOfBoundsException("队列已满的异常");
        }
        elementData[rear++] = element;
    }
    //移除队列
    public T remove()
    {
        if (empty())
        {
            throw new IndexOutOfBoundsException("空队列异常");
        }
        //保留队列的rear端的元素的值
        T oldValue = (T)elementData[front];
        //释放队列的rear端的元素
        elementData[front++] = null;
        return oldValue;
    }
    //返回队列顶元素，但不删除队列顶元素
    public T element()
    {
        if (empty())
        {
            throw new IndexOutOfBoundsException("空队列异常");
        }
        return (T)elementData[front];
    }
    //判断顺序队列是否为空队列
    public boolean empty()
    {
        return rear == front;
    }
    //清空顺序队列
    public void clear()
    {
        //将底层数组所有元素赋为null
        Arrays.fill(elementData , null);
        front = 0;
        rear = 0;
    }
    public String toString()
    {
        if (empty())
        {
            return "[]";
        }
        else
        {
            StringBuilder sb = new StringBuilder("[");
            for (int i = front  ; i < rear ; i++ )
            {
                sb.append(elementData[i].toString() + ", ");
            }
            int len = sb.length();
            return sb.delete(len - 2 , len).append("]").toString();
        }
    }
}

2.循环队列（顺序结构存储实现）

import java.util.Arrays;
public class LoopQueue<T>
{
    private int DEFAULT_SIZE = 10;
    //保存数组的长度。
    private int capacity;
    //定义一个数组用于保存循环队列的元素
    private Object[] elementData;
    //保存循环队列中元素的当前个数
    private int front = 0;
    private int rear = 0;
    //以默认数组长度创建空循环队列
    public LoopQueue()
    {
        capacity = DEFAULT_SIZE;
        elementData = new Object[capacity];
    }
    //以一个初始化元素来创建循环队列
    public LoopQueue(T element)
    {
        this();
        elementData[0] = element;
        rear++;
    }
    /**
     * 以指定长度的数组来创建循环队列
     * @param element 指定循环队列中第一个元素
     * @param initSize 指定循环队列底层数组的长度
     */
    public LoopQueue(T element , int initSize)
    {
        this.capacity = initSize;
        elementData = new Object[capacity];
        elementData[0] = element;
        rear++;
    }
    //获取循环队列的大小
    public int length()
    {
        if (empty())
        {
            return 0;
        }
        return rear > front ? rear - front
            : capacity - (front - rear);
    }
    //插入队列
    public void add(T element)
    {
        if (rear == front
            && elementData[front] != null)
        {
            throw new IndexOutOfBoundsException("队列已满的异常");
        }
        elementData[rear++] = element;
        //如果rear已经到头，那就转头
        rear = rear == capacity ? 0 : rear;
    }
    //移除队列
    public T remove()
    {
        if (empty())
        {
            throw new IndexOutOfBoundsException("空队列异常");
        }
        //保留队列的rear端的元素的值
        T oldValue = (T)elementData[front];
        //释放队列的rear端的元素
        elementData[front++] = null;
        //如果front已经到头，那就转头
        front = front == capacity ? 0 : front;
        return oldValue;
    }
    //返回队列顶元素，但不删除队列顶元素
    public T element()
    {
        if (empty())
        {
            throw new IndexOutOfBoundsException("空队列异常");
        }
        return (T)elementData[front];
    }
    //判断循环队列是否为空队列
    public boolean empty()
    {
        //rear==front且rear处的元素为null
        return rear == front
            && elementData[rear] == null;
    }
    //清空循环队列
    public void clear()
    {
        //将底层数组所有元素赋为null
        Arrays.fill(elementData , null);
        front = 0;
        rear = 0;
    }
    public String toString()
    {
        if (empty())
        {
            return "[]";
        }
        else
        {
            //如果front < rear，有效元素就是front到rear之间的元素
            if (front < rear)
            {
                StringBuilder sb = new StringBuilder("[");
                for (int i = front  ; i < rear ; i++ )
                {
                    sb.append(elementData[i].toString() + ", ");
                }
                int len = sb.length();
                return sb.delete(len - 2 , len).append("]").toString();
            }
            //如果front >= rear，有效元素为front->capacity之间、0->front之间的
            else
            {
                StringBuilder sb = new StringBuilder("[");
                for (int i = front  ; i < capacity ; i++ )
                {
                    sb.append(elementData[i].toString() + ", ");
                }
                for (int i = 0 ; i < rear ; i++)
                {
                    sb.append(elementData[i].toString() + ", ");
                }
                int len = sb.length();
                return sb.delete(len - 2 , len).append("]").toString();
            }
        }
    }
}

3.队列的链式存储结构及实现

public class LinkQueue<T>
{
    //定义一个内部类Node，Node实例代表链队列的节点。
    private class Node
    {
        //保存节点的数据
        private T data;
        //指向下个节点的引用
        private Node next;
        //无参数的构造器
        public Node()
        {
        }
        //初始化全部属性的构造器
        public Node(T data ,  Node next)
        {
            this.data = data;
            this.next = next;
        }
    }
    //保存该链队列的头节点
    private Node front;
    //保存该链队列的尾节点
    private Node rear;
    //保存该链队列中已包含的节点数
    private int size;
    //创建空链队列
    public LinkQueue()
    {
        //空链队列，front和rear都是null
        front = null;
        rear = null;
    }
    //以指定数据元素来创建链队列，该链队列只有一个元素
    public LinkQueue(T element)
    {
        front = new Node(element , null);
        //只有一个节点，front、rear都指向该节点
        rear = front;
        size++;
    }
    //返回链队列的长度
    public int length()
    {
        return size;
    }
    //将新元素加入队列
    public void add(T element)
    {
        //如果该链队列还是空链队列
        if (front == null)
        {
            front = new Node(element , null);
            //只有一个节点，front、rear都指向该节点
            rear = front;
        }
        else
        {
            //创建新节点
            Node newNode = new Node(element , null);
            //让尾节点的next指向新增的节点
            rear.next = newNode;
            //以新节点作为新的尾节点
            rear = newNode;
        }
        size++;
    }
    //删除队列front端的元素
    public T remove()
    {
        Node oldFront = front;
        front = front.next;
        oldFront.next = null;
        size--;
        return oldFront.data;
    }
    //访问链式队列中最后一个元素
    public T element()
    {
        return rear.data;
    }
    //判断链式队列是否为空队列
    public boolean empty()
    {
        return size == 0;
    }
    //清空链队列
    public void clear()
    {
        //将front、rear两个节点赋为null
        front = null;
        rear = null;
        size = 0;
    }
    public String toString()
    {
        //链队列为空链队列时
        if (empty())
        {
            return "[]";
        }
        else
        {
            StringBuilder sb = new StringBuilder("[");
            for (Node current = front ; current != null
                ; current = current.next )
            {
                sb.append(current.data.toString() + ", ");
            }
            int len = sb.length();
            return sb.delete(len - 2 , len).append("]").toString();
        }
    }
}